Identification and Characterization of β-Lathyrin, an Abundant

Jul 27, 2018 - Key Laboratory of Soybean Biology at the Chinese Ministry of Education, Northeast Agricultural University , Harbin , Heilongjiang 15003...
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Agricultural and Environmental Chemistry

Identification and characterization of #-lathyrin, an abundant glycoprotein of grass pea (Lathyrus sativus L.), as a potential allergen Quanle Xu, Bo Song, Fengjuan Liu, Yaoyao Song, Peng Chen, Shanshan Liu, and Hari B. Krishnan J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.8b02314 • Publication Date (Web): 27 Jul 2018 Downloaded from http://pubs.acs.org on July 31, 2018

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Identification and characterization of β-lathyrin, an abundant glycoprotein of grass

2

pea (Lathyrus sativus L.), as a potential allergen

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Quanle Xu1,2, Bo Song2,3, Fengjuan Liu1, Yaoyao Song1, Peng Chen1, Shanshan Liu3, and

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Hari B. Krishnan2,4*

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1

College of Life Sciences, Northwest A&F University, Yangling, Shaanxi 712100, China

8

2

Plant Science Division, University of Missouri, Columbia, MO 65211

9

3

Key Laboratory of Soybean Biology at the Chinese Ministry of Education, Northeast

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Agricultural University, Harbin, China

11

4

Plant Genetics Research, USDA-Agricultural Research Service, Columbia, MO 65211

12 13 14 15 16 17 18

Corresponding author’s address:

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*Dr. Hari B. Krishnan

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Plant Genetics Research Unit

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USDA-ARS, 108 Curtis Hall

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University of Missouri, Columbia, MO 65211

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E-mail: [email protected]

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ABSTRACT: Grass pea, a protein-rich, high-yielding and drought-tolerant legume, is

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utilized as food and livestock feed in several tropical and subtropical regions of the

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world. The abundant seed proteins of grass pea are salt-soluble globulins, which can be

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separated into vicilins and legumins. In many other legumes, the members of vicilin seed

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proteins have been identified as major allergens. However, very little information is

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available on the allergens of grass pea. In this study, we have identified an abundant 47

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kDa protein from grass pea which was recognized by IgE antibodies from sera drawn

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from several peanut-allergic patients. The IgE-binding 47 kDa protein was partially

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purified by affinity chromatography on a Con-A sepharose column. MALDI-TOF mass

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spectrometry analysis of the 47 kDa grass pea protein revealed sequence homology to 47

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kDa vicilin from pea and Len c 1 from lentil. Interestingly the grass pea vicilin was found

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to be susceptible to pepsin digestion in vitro. We have also isolated a cDNA encoding the

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grass pea 47 kDa vicilin (β-lathyrin) and the deduced amino acid sequence revealed

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extensive homology to several known allergens including those from peanut and soybean.

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A homology model structure of the grass pea β-lathyrin, generated using the X-ray

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crystal structure of the soybean β-conglycinin β-subunit as a template, revealed potential

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IgE-binding epitopes located on the surface of the molecule. The similarity in the three-

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dimensional structure and the conservation of the antigenic epitopes on the molecular

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surface of vicilin allergens explains for the IgE-binding cross-reactivity.

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KEYWORDS: Allergen; β-lathyrin; grass pea; storage protein

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INTRODUCTION

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Grass pea (L. sativus L.) is an important legume that is cultivated as food and forage crop

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in North Africa, Near East, Western Asia and the Indian subcontinent.1-3 This under-

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utilized legume can withstand harsh environmental conditions including drought and

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flooding. On account of these characteristics grass pea is often grown in drought-prone

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areas.3 Like other legumes, grass pea seeds are relatively rich in protein (about 20% of

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the seed dry weight) and can serve as economic source of highly nutritious and balanced

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source of human dietary protein. However, its use as a dietary protein source is limited

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due to the presence of the neurotoxic nonprotein amino acid β-N-oxalyl-L-α,β-

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diaminopropionic acid (β-ODAP). Prolonged consumption of grass pea as a sole protein

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source leads to paralysis of lower limbs, a condition referred to as lathyrism.4,5 The

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concerted efforts of international research organizations have resulted in the development

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of improved varieties that contain lower levels of β-ODAP.6,7 Such improved grass pea

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varieties are vital for reducing incidence of lathyrism among subsistence poor farmers in

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counties that regularly have harsh environmental conditions.

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Unlike the major legumes, little is known about the seed storage proteins of grass pea.

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Though several early studies have been reported,8-11 it was only in the year 2000 a

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detailed biochemical investigation of grass pea seed proteins was initiated.12 It was

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reported that the protein content of grass pea was about 20% of the seed dry weight.

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Globulin and albumin fractions represented 60 and 30% of the total seed proteins,

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respectively. The globulins were grouped into α-lathyrin, β-lathyrin, and γ-lathrin. The

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α-lathyrin, the most abundant globulin of grass pea seeds, is represented by three proteins

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that were further subjected to proteolytic processing resulting in larger and small

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subunits, similar to the 11S globulins of other legumes.12 The β-lathyrin (7S globulin) is

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made up of several polypeptides of different molecular weights (14-66 kDa). At least

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two of the subunits of β-lathyrin are glycosylated and one of them contains a disulfide

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bond.12 Thus, the seed storage protein composition of grass pea is similar to that of other

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well characterized legumes, though slight changes in the sedimentation coefficients are

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evident.12 Immunological studies further confirm the similarity between lupin conglutins

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and grass pea lathyrins in subunit and polypeptide composition.12

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Allergic reactions to legume seed protein (especially peanut, soybeans, lentil,

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chickpea and pea) have been widely reported in the literature.13 In Mediterranean and

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Asian countries, allergic reactions to the ingestion of lentil, chickpea, and pea are

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prevalent. For example, 10% of the Spanish pediatric population was reported to be

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allergic to lentil proteins.14 The major allergen of lentil has been identified as vicilin (Len

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c 1), a member of 7S globulin.15 Interestingly, the 7S globulin members are also most

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dominant allergens in peanut and soybean.16-18 In contrast to other legumes, very little is

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known about the allergenic potential of grass pea seed proteins. In sensitized individuals,

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grass pea can induce symptoms of food allergy as well as occupational-induced allergies

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such as bronchial asthma, rhinoconjunctivitis, facial oedema, and generalized urticaria.19-

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23

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present in the blood from grass pea allergic patients.21 Subsequently, a 24 kDa allergenic

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protein from grass pea was purified and crystallized.23 This allergenic protein was

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identified as a member of 2S albumin and showed 85% sequence homology with the 2S

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albumin of Pisum sativum. Since grass pea is often the only crop that poor farmers are

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able to depend on to survive in extremely drought-prone growing regions, it is critical to

Three polypeptides (sized 46, 28, and 21 kDa) were recognized by IgE antibodies

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identify and characterize potential allergens present in seeds of grass pea. Here, we have

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identified and characterized a 47 kDa IgE-binding protein from grass pea.

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MATERIALS AND METHODS

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Seed materials. Dry seeds of L. sativus cv. LZ(2) were harvested in 2016 from an

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experimental farm of Northwest A&F University, China. Seed of soybean (Glycine max

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cv Williams 82) were obtained from Missouri Crop Improvement Association, Columbia,

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MO. Seeds of Spanish peanut (Arachis hypogaea) was purchased from a local grocery

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store, Columbia, MO. All seeds were stored at 4 °C until used.

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Patient’s Sera. Sera from eight patients with documented allergy to peanut were

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obtained from IBT Laboratories (Lenexa, KS). ImmunoCAP® FEIA assay, which has been

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approved for diagnostic use by the U.S. Food and Drug Administration, revealed 175.0 - 50.9

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kU/L peanut-specific IgE in the sera of these patients. Clinical symptoms of these

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patients ranged from impaired breathing, itchy skin, nausea and runny nose. Four of eight

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peanut allergic patients also contained IgE specific to soybean flour, as determined by the

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ImmunoCAP® FEIA assay. Serum from a health individual with no known history of

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allergy was also acquired from IBT Laboratories.

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Protein

isolation

and

Sodium

Dedecyl

Sulfate−Polyacrylamide

Gel

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Electrophoresis (SDS-PAGE). Dry seeds were ground to a fine powder in a mortar and

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pestle. Then 20 mg of seed powder was transferred to a 2 mL microcentrifuge tube and 1

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mL of sodium dodecyl sulfate (SDS)-sample buffer (60 mM Tris-HCl, pH 6.8, 2% SDS

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(w/v), 10% glycerol (v/v), and 5% 2-mercaptoethanol (v/v)) was added. The contents of

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each tube were vigorously vortexed for 10 min at room temperature followed by boiling

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for 5 min. Each sample was then subjected to centrifugation at 15,800g for 5 min. The

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clear supernatant was collected and designated as total seed protein fraction. Seed

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proteins were then resolved by SDS-PAGE gels using a Hoeffer SE 250 Mini-Vertical

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electrophoresis apparatus (GE Healthcare, Pittsburg, PA, U.S.A.), and ~50 µg of seed

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proteins were loaded onto a 13.5% acrylamide gel. Separation of proteins was achieved

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with a constant 20 mA/gel until the tracking dye reached the bottom of the gel. Gels were

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then removed from the cassette and stained overnight with Coomassie Blue R-250.

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Immunoblot Analysis. Seed proteins were first resolved by SDS-PAGE and then

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electrophoretically transferred to nitrocellulose membranes (Protran, Schleicher &

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Schuell Inc., Keene, NH) as previously described.24 Membranes were blocked with 5%

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non-fat milk in Tris-buffer saline (TBS, pH 7.3) for 1 h at room temperature. Following

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this step, the nitrocellulose membranes were incubated in a 1:500 dilution of serum from

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peanut allergic patients (IBT Laboratories, Lenexa, KS) overnight at room temperature

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with gentle rocking. After washing (four times with TBS containing 0.05% Tween-20,

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TBST, 10 min/each), each membrane was incubated for 2 h in a 1:5000 dilution of goat

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anti-human IgE-horseradish peroxidase conjugate antibody (Biosource, Camerillo, CA).

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IgE-binding polypeptides were then detected using an enhanced chemiluminescent

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substrate (Super Signal West Pico trial kit; Pierce Biotechnology, Rockford, IL)

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according to the manufacturer's protocol.

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Con-A-Sepharose Affinity Chromatography. Five grams of dry grass pea seed was

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ground to a fine powder using a mortar and pestle. To the seed powder, 50 mL of 50 mM

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Tris-HCl, pH 7.5, 150 mM NaCl and 0.1 mM phenylmethylsulfonyl fluoride was added.

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The mixture was then placed in an orbital shaker (160 rpm, 30 C) for 30 min. The slurry

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was clarified by centrifugation at 13,320g for 20 min. To the resulting clear supernatant

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solid, (NH4)2SO4 was added gradually to achieve 70% saturation. The precipitated

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proteins were then recovered by centrifugation as described previously. The resulting

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protein pellet was resuspended in 20 mL of dialysis buffer (100 mm sodium phosphate

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(pH 6.5), 1 mM MnCl2, 1 mM CaCl2, 1 mM MgCl2, 1 mM DTT, and 0.5 M NaCl), and

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the concentrated protein solution was dialysed overnight against the same buffer at room

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temperature. The dialyzed protein solution was then loaded onto a column (1 x 16 cm) of

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Concanavalin A-Sepharose (Sigma Chemicals, St. Louis, MO), and the column was

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washed extensively with dialysis buffer and then subjected to a step gradient of 0 to 250

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mM α-methyl manoside in dialysis buffer. Fractions of 5 mL were collected, and

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alternative fractions were subjected to SDS-PAGE analysis.

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MALDI-TOF Mass Spectrometry Analysis. Partially purified grass pea vicilin

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fractions obtained from ConA-sepharose chromatography were subjected to SDS-PAGE.

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The 47 kDa IgE-binding protein was excised from the acrylamide gel, washed in distilled

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water and then destained in a 50% solution of acetonitrile (v/v) containing 25 mM

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ammonium bicarbonate. After destaining, a 100% acetonitrile wash was performed, and

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the 47 kDa protein was digested with 20 µL (10 µg/mL) of modified porcine trypsin in 25

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mM ammonium bicarbonate (Promega, Madison, WI). The peptides resulting from

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tryptic digestion were then analyzed by mass spectroscopy as previously described.24

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Enzymatic Deglycosylation of Grass Pea Allergen. Deglycosylation of the grass

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pea 47 kDa vicilin was carried out using GlycoPro Enzymatic Deglycosylation Kit

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(Prozyme, Hayward, CA) per the manufacturer's instructions. The kit includes N-

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Glycanase,

sialidase

A,

O-Glycanase,

β(1-4)

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Glactosidase

and

β-N-

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Acetylglucosaminidase, which removes all N-linked glycans and simple O-linked glycans

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from glycoproteins. About 100 µg of ConA-sepharose purified vicilin fraction was

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denatured and deglycosylated following the manufacturer’s protocol. The efficiency of

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the deglycosylation was verified by a shift in protein mobility on a 10% SDS-PAGE gel.

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Protease Digestion of Grass Pea Allergen. The stability of grass pea 47 kDa vicilin

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to pepsin digestion was carried out as described previously.25,26 About 20 µg of the

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ConA-sepharose purified vicilin fraction was digested with pepsin (Sigma), at one of

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several time intervals at 37 °C in simulated gastric fluid (SGF): Aliquots of samples were

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removed after 30 seconds, 1 min, 5 min, 30 min and 60 min. To these aliquots 30 µL of

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Na2CO3 (200 mM), 34 µL of 6× SDS−sample treatment buffer (350 mM Tris-HCl, 10%

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SDS, 30% glycerol, and 175 mM bromophenol blue, pH 6.8), and 10 µL of β-ME were

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added, mixed, and then boiled for 5 min. Controls included pepsin in SGF without added

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protein or protein samples in SGF that did not contain pepsin. The generation of

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proteolytic fragments due to pepsin digestion was examined by SDS-PAGE and

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immunoblot analysis.

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Homology-Model of the 47 kDa vicilin from Grass Pea. Utilizing the three-

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dimensional structure model of the homotrimeric soybean β-conglycinin β-subunit27 as a

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template, a homology model of the grass pea 47 kDa vicilin was build. The homology

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model was generated as a homotrimer and energy minimized using the Swiss-Model

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website (http://swissmodel.expasy.org//SWISS-MODEL.html).28 The stereochemistry

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and overall quality of the homology model were examined via the program Procheck.29

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RESULTS AND DISCUSSION

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Immunoglobulin E cross-reactivity of grass pea seed proteins with sera from

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peanut allergic persons. Allergy to peanuts is a major public health concern in the

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United States. It has been estimated that about 2% of US children have peanut allergy.

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The official allergen database (htpp:/www.allergen.org) has recognized twelve peanut

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allergens. Two of them belong to the cupin (Ara h 1, Ara h 3), four to the prolamin (Ara h

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2, Ara h 6, Ara h 7, Ara h 9), one to the profiling (Ara h 5), one to the Bet v 1 (Ara h 8),

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two to the glycosyl transferease GT-C (Ara h 10, Ara h 11) and two to the scorpion-like

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knottin (Ara h 12, Ara h 13) superfamilies.30 Interestingly, peanut allergic patients also

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develop clinically relevant sensitization to several different legumes such as soybean,

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pea, lupin and lentil.30 Allergic reactions to consumption of legumes such as lupin, lentil,

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chickpea and pea are commonly reported in Mediterranean and Asian countries. Serum

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from persons with peanut and/or soybean allergy have been used to demonstrate IgE

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cross-reactivity with seed proteins from some of these legumes.30-33 To verify if the sera

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from peanut allergy patients can cross-react with seed proteins of grass pea, we

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performed immunoblot analysis utilizing serum from 8 different patients with peanut

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allergies (Figure 1). As expected, peanut allergy sera from all the eight patients

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recognized several allergens from the peanut protein extract. The sera also cross-reacted

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with both soybean and grass pea proteins. Serum from 6/8 peanut allergic subjects

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strongly reacted against grass pea proteins while no cross-reactivity was detected from

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the sera from another two patients. In the case of grass pea, a protein with apparent

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molecular weight of 47 kDa cross-reacted with sera from most of peanut allergic subjects.

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In addition to 47 kDa immunodominant protein, Ig-E cross-reactivity against few other

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proteins were also detected from grass pea protein extract (Figure 1).

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Identification of grass pea 47 kDa IgE binding protein as β -lathyrin. The molecular

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weight and relatively high abundance of grass pea 47 kDa IgE binding protein suggested

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that this protein may belong to 7S vicilin family. Native vicilin are present as trimers with

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molecular weight ranging from 150 – 210 kDa and are encoded by multigene families.34

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Vicilins fall into two groups, the first made of subunits with molecular weights of

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approximately 50 kDa, while the second group includes 60 – 70 kDa proteins.34 These

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two group of proteins are largely homologous except that the first group bears a N-

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terminal extension of variable length which results in higher molecular weights.34 Vicilin

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are subjected to posttranslational modifications (glycosylation and proteolysis) leading to

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variation in their subunit molecular weights. Most of these glycosylated proteins are

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potential allergens and are resistant to digestion and food processing.35 In the case of

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grass pea, it has been previously shown that some grass pea 7S globulins are

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glycoproteins.12 Therefore, we utilized Con-A Sepharose affinity chromatography to

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purify the 47 kDa IgE-binding protein from grass pea. When Tris-buffered saline (TBS)

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extracted grass pea proteins were loaded on a Con-A column, several of the abundant

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seed proteins did not bind to the Con-A column (Figure 2).

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A 47 kDa protein, which was initially retained on the Con-A column, was eluted with

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approximately 50 mM α-methylmanoside. SDS-PAGE analysis of 50 mM α-

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methylmanoside eluted protein fraction revealed an abundant protein with an estimated

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molecular weight of 47 kDa and several other less abundant proteins (Figure 2).

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Antibodies raised against soybean β-subunit of β-conglycinin, a glycoprotein, strongly

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reacted against the 47 kDa protein (Figure 2). Similarly, western blot analysis using

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pooled serum from peanut allergy patients also reacted strongly against the 47 kDa

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protein. We excised the IgE-binding SDS-PAGE separated 47 kDa protein, digested it

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with trypsin and subjected it to MALDI-TOF-MS (Table 1). Using Mascot, the

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empirically determined mass-to-charge ratios of the peptides were then compared to

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known peptides present in the National Center for Biotechnology Information

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nonreduandant database. Three peptides from the 47 kDa protein gave statistically

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significant protein scores for matches with Len c 1, a major allergen from lentil seeds

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(Table 1). Similarly, the same three peptides gave significant MOWSE score matches

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against a hypothetical protein from clover (Trifolium subterraneum) and pea vicilin

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(Table 1). Blast analysis of this hypothetical protein revealed significant amino acid

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sequence homology to vicilins from several legumes: Medicago truncatula, Cicer

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arietinum, Pisum sativum, Glycine max as well as convicilins from Vicia narbonensis,

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Pisum sativum and Lotus japonicas.

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Our laboratory is also analyzing the expression profiles of grass pea genes by RNA

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sequencing and have generated in-house grass pea transcriptome database. A search of

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this database with Medicago truncatula vicilin sequences resulted in the identification of

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a transcript encoding the 47 kDa β-lathyrin. The deduced amino acid sequence of

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identified cDNA encodes a truncated protein with apparent molecular weight of 42 kDa.

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Bioinformatic analysis indicated that the grass pea β-lathyrin belongs to the cupin

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superfamily (Figure 3). Blast analysis revealed that grass pea β-lathyrin shares extensive

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amino acid sequence homology with 47 kDa vicilin from Pisum sativum and Medicago

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truncatula, a hypothetical protein from Medicago truncatula, allergen Len c 1 from Lens

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culinaris and several other legume 7S globulin family members. (Supplementary Figure

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1). A search of the amino acid sequence of the 47 kDa grass pea β-lathyrin revealed exact

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matches to several trypsin digested peptide sequences generated from the 47 kDa IgE-

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binding peptide (Figure 3). Based on these analyses, the 47 kDa IgE binding protein from

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grass pea has been identified as β-lathyrin.

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Carbohydrate moiety of the grass pea β-lathyrin is not responsible for the

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immunoreactivity.

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Glycosylation is a common feature to many food allergens.35 It is believed that glycan

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moieties

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immunogenicity.36,37 Even though antibodies specific to carbohydrate determinants are

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commonly found, they appear not to have clinical relevance.35 However, anaphylaxis

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induced by carbohydrate epitopes has been reported.38 To evaluate the role of the

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carbohydrate moiety in IgE-binding, partially purified grass pea β-lathyrin was subjected

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to deglycosylation (Figure 4).

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The product of enzymatic deglycosylation were then analyzed by SDS-PAGE which

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revealed two prominent proteins of 47 and 45 kDa proteins. The 47 kDa and 45 kDa

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proteins presumably represent the glycosylated and the deglycosylated form of β-lathyrin

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(Figure 4). The removal of the carbohydrate moiety caused a decrease in molecular

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weight of the β-lathyrin; protein corresponding to the same size was not detected in the

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control lane (Figure 4). Western blot analysis, using pooled serum from peanut allergy

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patients, revealed IgE binding to both native and deglycosylated proteins (Figure 4). This

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observation indicates that the carbohydrate moiety of β-lathyrin may not be critical for

273

IgE-binding. Interestingly, antibodies raised against the soybean beta subunit of β-

may exert

a stabilizing effect

on

protein structure and

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conglycinin (which is also a glycoprotein) recognized the native but not the

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deglycosylated β-lathyrin (Figure 4). The reason why soybean β-conglycinin specific

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antibodies did not react against the deglycosylated β-lathyrin is not clear. It is likely that

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that soybean β-conglycinin antibodies are mainly targeted to the core oligosaccharides

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covalently attached to β-conglycinin and β-lathyrin. Soybean β-conglycinin contains N-

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linked mannose-containing core oligosaccharides39 and the strong cross-reactivity of the

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soybean β-conglycinin antibodies against the β-lathyrin indicate the presence of similar

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core oligosaccharides bound to the grass pea β-lathyrin.

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Grass pea β -lathyrin is susceptible to pepsin digestion. It is generally believed that

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food allergens can survive the acidic gastric environment as they are resistant to pepsin

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digestion.40 On account of this, digestion assays in simulated gastric fluid (SGF) are

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routinely employed to predict the allergenic potential of food proteins. An examination of

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the stability of 25 different food proteins to pepsin digestion demonstrated that allergens

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have greater stability to protease digestion.26 To examine if grass pea β-lathyrin is also

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stable to digestion, partially purified proteins were subjected to pepsin digestion in vitro

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(Figure 5).

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Incubation of the β-lathyrin with pepsin resulted in rapid breakdown of the mature β-

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lathyrin and the appearance of proteolytic products within 30 seconds. By the end of 60

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min the β-lathyrin was completely digested (Figure 5). Western blot analyses using

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antibodies raised against soybean β-conglycinin antibodies or pooled serum from peanut

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allergy patients failed to detect either mature convicilin or the breakdown products after 1

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min pepsin digestion (Figure 5). Earlier studies have shown that soybean Kuntiz trypsin

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inhibitor (KTi), a potent allergen capable of inducing food anaphylaxis,41 has

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demonstrated remarkable stability to both thermal and acid denaturation.42 For

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comparison, we examined the stability of KTi to pepsin digestion under the experimental

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conditions used to test the stability of β-lathyrin. Unlike grass pea β-lathyrin, soybean

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KTi was stable even up to 60 min digestion using pepsin (Figure 5). However, resistance

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to pepsin digestion alone can’t be used as criteria for predicting the allergenicity of a

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protein. Previous studies have established that some well-known allergens are also

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digested by pepsin within a few seconds.43,44

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Identification of IgE-binding Epitopes on the Modeled Structure of Grass Pea β -

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Lathyrin. A homology model structure of the grass pea β-lathyrin was generated using

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the X-ray crystal structure of the soybean β-conglycinin β-subunit (SWISS-MODEL

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Template Library ID: 1ipk.1.A) core domain (residues 49-397) as a template.45 The core

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domain of the soybean β-conglycinin β-subunit and grass pea β-lathyrin share 59.4%

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amino acid sequence identity over the region included in the X-ray structure. The Global

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Model Quality Estimation (GMQE) and Qualitative Model Energy ANalysis (QMEAN)

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scores were 0.75 and -1.38, respectively, which suggested a good quality of homology

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model structure. Figure 6 shows the ribbon diagrams of the grass pea β-lathyrin. The

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modeled structure of grass pea β-lathyrin is consistent with those of vicilin family

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members.45 Grass pea β-lathyrin model structure is very similar to the structures of the 7S

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seed storage proteins from soybean,45 peanut,46,47 adzuki bean,48 kidney bean,50 jack

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bean,51 and pecan.52 IgE-binding epitopes within grass pea β-lathyrin was also identified

317

utilizing AlgPred server.53 This computational method identified a single IgE binding

318

epitope (ELHLLGFGIN), and predicted grass pea β-lathyrin as an allergen. Similarly, a

319

BLAST search of β-lathyrin against 2890 allergen-representative peptides also confirmed

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the

existence

of

an

allergenic-representative

321

(PHFNSKAMVIVVVNKGTGNLELVA) in β-lathyrin. The NetMHCⅡ 2. 2 Server was

322

used to measure the binding ability between β -lathyrin and MHC II, and results suggested

323

that β-laythrin contains nine potential IgE-binding epitopes. These potential epitopes,

324

named T1-T9, are highlighted by different colors in one the monomers (Figure 6).

325

Epitopes T2, T6, T7 and T8 were located in the conserved domain of Cupin superfamily

326

proteins (Fig 3). Epitope T8 is common in other legume allergens, such as Ara h 1, Gly m

327

5, Len c 1, Lup an 1, Pis s 2, and Vig r 2. In contrast T4 is only shared with Len c1.

328

Epitope T2 was specific for β-lathyrin and was not found in other species. The

329

conservation of highly similar IgE-binding epitopes among different legume allergens

330

may explain the IgE-cross-reactivity of sera from peanut sensitive patients to grass pea β-

331

lathyrin.

332

In conclusion, we have identified grass pea β-lathyrin as a potential allergenic protein.

333

Most of the IgE-binding epitopes of β-lathyrin identified in this study were found to be in

334

common to other legume allergens and located on the predicted protein surface. The

335

conservation of IgE-binding epitopes among several distinct legume allergens may be

336

responsible for the observed phenomenon of cross-reactivity. Additional studies, leading

337

to a better understanding of allergenicity potential of grass pea β-lathyrin, will aid in the

338

design and development of effective immunotherapy strategies for the diagnosis and

339

treatment of grass pea-related food allergy.

340 341

ACKNOWLEDGEMENTS

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This research was supported by the USDA-Agricultural Research Service, the Chinese

343

Universities Scientific Fund (2014YB040), China Postdoctoral Science Foundation

344

(2016M590975),

345

(2016BSHEDZZ119), P.R. China. Mention of a trademark, vendor, or proprietary

346

product does not constitute a guarantee or warranty of the product by the USDA and does

347

not imply its approval to the exclusion of other products or vendors that may also be

348

suitable.

Postdoctoral

Science

Foundation

of

Shaanxi

province

349 350

Supporting Information

351

Amino acid sequence alignment of grass pea β-lathyrin with several other legume 7S

352

globulin family members, and phylogenetic analysis of known food allergens.

353

REFERENCES

354

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Lathyrus sativus (grass pea) and its neurotoxin ODAP. Phytochemistry 2006, 67,

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syndrome. J. Allergy Clin. Immunol. 2003, 111, 634–639.

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protein of mung bean. Acta Crystallogr. Sect D Biol. Crystallogr. 2006, 62, 824–

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structure and the genetic engineering of seed storage proteins. J. Mol.

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Biol. 1994, 238, 748–776.

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in two crystal forms at 2.1 and 2.0 A resolution. Acta Crystallogr. Sect D Biol.

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Crystallogr. 2000, 56, 411–420.

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(52) Zhang, Y.; Lee, B.; Du, W. X.; Lyu, S. C.; Nadeau, K. C.; Grauke, L. J.; Wang, S.;

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Fan, Y.; Yi, J.; McHugh, T. H. Identification and characterization of a new pecan

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[Carya illinoinensis (Wangenh.) K. Koch] allergen, Car i2. J. Agric. Food Chem.

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(53) Saha, S.; Raghava, G. P. S. (2006) AlgPred: prediction of allergenic proteins and mapping of IgE epitopes. Nucleic Acids Res. 2006, 34, W202–W209.

504 505 506 507

Figure captions

508

Figure 1. SDS-PAGE and IgE cross-reactivity of grass pea proteins. Panel A: Total seed

509

proteins extracted from seeds of peanut (lane 1); soybean (lane 2) and grass pea (lane 3)

510

were separated by 13.5 % SDS-PAGE gel. Resolved proteins were detected by staining

511

the gel with Coomassie Blue. Panel B – I: Western blot analysis showing cross-reactivity

512

of grass pea proteins with sera from patients with peanut allergy. Each panel was

513

incubated with sera from individual peanut-allergic patients. Cross-reacting proteins were

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identified using goat anti-human IgE-horseradish peroxidase conjugate antibody followed

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by chemiluminescent detection. Lane M contains the molecular weight markers whose

516

sizes in kDa are shown on the left side of the figure.

517

Figure 2. Purification of 47 kDa IgE-binding grass pea protein. Panel A: Grass pea

518

protein were separated by 10 % SDS-PAGE and visualized by staining with Coomassie

519

Blue (Panel A) or electrophoretically transferred to nitrocellulose membrane and

520

incubated with pooled sera from several peanut-allergic patients (Panel B), or soybean β-

521

conglycinin antisera (Panel C). Immunoreactive proteins were identified using goat anti-

522

human IgE-horseradish peroxidase conjugate or anti-rabbit IgG-horseradish peroxidase

523

conjugate followed by chemiluminescent detection. Lane 1, TBS extracted proteins from

524

grass pea; lane 2, Con-A Sepharose unbound proteins; lane 3, 50 mM α-methylmanoside

525

eluted proteins. The numbers on the left side of the figure indicates the sizes of protein

526

standards in kDa.

527

Figure 3. Schematic representation of grass pea β-lathyrin coding sequences showing

528

Cupin-1 domains (Panel A) and amino acid regions that match with the trypsin digested

529

peptide sequences from the 47 kDa IgE-binding protein (Panel B).

530

Figure 4. SDS-PAGE/immunoblot analysis of enzymatically deglycosylated grass pea

531

β-lathyrin. Partially purified native (lane 1) and deglycosylated (lane 2) grass pea β-

532

lathyrin were separated by SDS-PAGE on a 10% gel and stained with Coomassie Blue

533

(Panel A), electrophoretically transferred to nitrocellulose membrane and challenged with

534

pooled sera from peanut allergic patients (Panel B), or soybean β-conglycinin antisera

535

(Panel C). Immunoreactive proteins were identified using goat anti-human IgE-

536

horseradish peroxidase conjugate or anti-rabbit IgG-horseradish peroxidase conjugate

537

followed by chemiluminescent detection. The numbers on the left side of the figure

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indicates the sizes of protein standards in kDa.

539

Figure 5. SDS-PAFE analysis of pepsin digested grass pea β-lathyrin and soybean

540

Kunitz trypsin inhibitor. Partially purified grass pea β-lathyrin (Panel A) and

541

commercially purchased Kuntiz trypsin inhibitor (Panel C) were digested with pepsin for

542

30 seconds, 1 min, 5 min, 30 min and 60 min and the products were fractionated by SDS-

543

PAGE on a 15% gel and stained with Coomassie Blue. Grass pea pepsin (Panel B)

544

digested β-lathyrin was electrophoretically transferred to nitrocellulose membrane and

545

challenged with pooled sera from peanut allergic patients. Immunoreactive proteins were

546

identified using goat anti-human IgE-horseradish peroxidase conjugate or anti-rabbit

547

IgG-horseradish peroxidase conjugate followed by chemiluminescent detection. The

548

numbers on the left side of the figure indicates the sizes of protein standards in kDa. Lane

549

M, protein markers; lane 1, pepsin in the absence of β-lathyrin, 0-time point; lane 2,

550

pepsin in the absence of β-lathyrin, 60 min time point; lane 3, β-lathyrin in the absence

551

pepsin, 0-time point; lane 4, β-lathyrin in the absence pepsin 60 min time point; lane 5, 6,

552

7, 8, and 9 contain pepsin digested protein samples that were removed at 30 seconds, 1

553

min, 5 min, 30 min and 60 min, respectively.

554

Figure 6. Homology-modeled structure of grass pea β-lathyrin. The trimeric grass pea β-

555

lathyrin is shown in a ribbon diagram with each monomer colored in red, blue and green

556

(Panel A). Potential IgE-binding epitopes of β -laythrin (Panel B). Panel C shows the

557

structural features corresponding to the 9 predicated antigenic epitopes, which are colored

558

following

the

color

code

shown

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Table 1. MALDI-TOF-MS identification of the grass pea 47 kDa protein.

Protein ID

MOWSE

Peptides Matched (Sequences) ---% Coverage

Allergen Len c 1.0101, partial

249

3 (3) ---10 %

255

3 (3) ---9%

GAU31893.1

(41-53) (245-256) (394-412)

205

3 (3) ---7%

VCLC PEA

(43-55) (66-74) (246-257)

(Lens culinarus) Hypothetical protein TSUD_270780

(Trifolium subterraneum) Vicilin OS

(Pisum sativum)

CAD87730.1

(16-28) (219-230) (368-386)

R.FQTLFENENGHIR.L K.SVSSESEPFNLR.N R.NFLAGEEDNVISQIQRPVK.E

71 80 98

R.FQTLFENENGHIR.L K.SVSSESEPFNLR.N R.NFLAGEEDNVISQIQRPVK.E

78 80 98

K.FQTLFENENGHIR.L K.IFENLQNYR.L K.SVSSESEPFNLR.S

78 48 80

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(Position) Peptide Matched

Peptide Score

Accession

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Figure 1

M

1

2

3

1

2

3

1

2

3

1

2

3

1

2

3

1

2

3

1

2

3

1

2

3

1

2

97 — 97 —

66 —

66 — 45 — 45 — 31 — 31 — 21 — 21 — 14 — 7—

A

B

C

D

E

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F

G

H

I

3

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Page # 26 Figure 2

M

1

2

3

1

2

3

1

2

3

97 — 66 —

45 —

31 —

21 —

A

B

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Figure 3

β-lathyrin Specific hits Superfamilies

YRLLEYKSKP HTLFLPQYTD ADFILVVLSG KAILTVLNSN DRNSFSLERG DTIKIPAGTI 70 80 90 100 110 120 AYLANRDDNE DLRVLDLAIP VNKPGQLQPF LLSGTQNQPS LLSGFSKKVL EAAFNTNYEE 130 140 150 160 170 180 IEKVLLEQQE QEPQHRRSLK DRRQEINEEN VIVKVSREQI EELSKNAKSS SKKSVSSESE 190 200 210 220 230 240 PFNLRSRNPI YSNKFGKFFE ITPEKNQQLQ DLDIFVNSVE IKEGSLLLPN YNSRAIVIVT 250 260 270 280 290 300 VNEGKGDFEL LGIRNENQRE ESDEEEEQEE ETSKQVQRYR AKLSPGDVFV IPAGHPVAIN 310 320 330 340 350 360 ASSNLNLIGF GINAENNQRN FLAGEEDNVI SQIQRPVKEL VFPGSSREVD KLLKNQRQSY 370 FANAQPLQRE

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Figure 4

M

1

2

1

2

1

2

97 —

66 —

45 —

31 —

A

B

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Figure 5

M 1

2

3

4

5

6

7

8

9

1

2

3

4

5

6

7

8

9

M 1

2

3

4

5

97 — 66 — 45 —

31 —

21 —

14 — 7—

A

B

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C

6

7

8

9

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Page # 30

Figure 6

A

90 °

B

C

T1:YRLLEYKSK; T2:LLSGFSKKV; T3:KVLLEQQEQ; T4:HRRSLKDRRQ; T5:IVKVSREQI; T6:LRSRNPIYSNKF; 7:TSKQVQRYRAKLS; T8:FLAGEEDNVISQIQRPV; T9:LLKNQRQSYFANA

180 °

180°

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