PROTEINS OF T H E TURKEY ERYTHROCYTE
MEMBRANE
Proteins of the Turkey Erythrocyte Membrane? Anne B. Caldwell
ABSTRACT: A new and simplified method is described for preparation of turkey erythrocyte membranes which are essentially devoid of supernatant or nuclear contamination, but retain catecholamine-sensitive adenylate cyclase activity. These membranes have been solubilized in sodium dodecyl sulfate and analyzed by polyacrylamide gel electrophoresis and the major protein components identified. T h e turkey erythrocyte membranes exhibit a protein profile very similar to that of the human erythrocyte membrane, but contain a protein component of apparent molecular weight of 50 000 which is not present in the human membranes. Three surface glycoprotein components of the turkey erythrocyte membranes
(apparent molecular weights of 90 000, 41 000, and 26 000) have been identified by periodic acid-Schiff staining of polyacrylamide gels and by cell surface labeling using lactoperoxidase followed by polyacrylamide gel electrophoresis. After deoxycholate solubilization of membranes prepared from iodinated cells, glycoproteins with molecular weights of 90 000 and 41 000 bind to an affinity column of concanavalin ASepharose 4B and elute upon application of methyl a - ~ mannopyrannoside. The lowest molecular weight glycoprotein component, however, does not bind to the insolubilized concanavalin A .
A l t h o u g h the avian erythrocyte has been known for many years to possess a catecholamine-sensitive adenylate cyclase (Davoren and Sutherland, 1963; Oye and Sutherland, 1966), which is not found in human erythrocyte, investigators have only recently begun to utilize this convenient system. Studies have largely concentrated on two related areas: the isolation and characterization of the membrane bound P-adrenoreceptor protein (Schramm et al., 1972; Bilezikian and Aurbach, 1973a,b; Cuatrecasas et al., 1974; Levitski et al., 1974; Atlas et al., 1974; Aurbach et al., 1974); and the regulation of the P-adrenoreceptor coupled adenylate cyclase (Bilezikian and Aurbach, 1974; Field, 1974; Puchwein et al., 1974; Steer and Levitski 1975a,b; Pfeuffer and Helmreich, 1975). In addition, a number of studies have explored the characteristics of sodium and potassium transport, a process in part regulated by C A M P ' (Gardner et a]., 1973, 1974a, 1974b, 1975). Despite the number of studies utilizing the turkey erythrocyte, published procedures for isolating pure avian erythrocyte membranes a r e tedious and time consuming (Zentgraf et al., 197 1; Bilezikian and Aurbach, 1973a; Puchwein et al., 1974) and in some studies sufficient data have not been presented to verify the purity of the final membrane product. Furthermore, detailed electrophoretic analysis of the proteins from purified avian erythrocyte membranes has not been attempted. Using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, Blanchet ( 1 974) has compared the profiles of solubilized nuclear and plasma membranes from adult and embryonic chicken erythrocytes. Jackson (1 975) has labeled the exterior surface of chicken erythrocytes with the cationic reagent p nitrophenyl N,N,N-trimethyl[ 12SI]iodotyrosinateand noted two surface glycoproteins on sodium dodecyl sulfate-polyacrylamide gels.
Identification of membrane protein and glycoprotein components is potentially important in further elucidating membrane events associated with catecholamine binding and regulation of adenylate cyclase activity. Well established techniques exist for the solubilization and separation of membrane proteins on polyacrylamide gels containing sodium dodecyl sulfate (Fairbanks et al., 1971). A number of workers have used lactoperoxidase-catalyzed iodination to label surface proteins containing exposed tyrosine residues in a variety of cell types (Marchalonis et al., 197 1; Kennel and Lerner, 1973; Czech and Lynn, 1973; Trosper and Levy, 1974; Nachman et al., 1973; Hubbard and Cohn, 1972, 1975; Tsai et al., 1973; Philips and Morrison, 1971; Reichstein and Blostein, 1973), followed by separation on polyacrylamide gel electrophoresis. Surface glycoproteins have also been identified by affinity chromatography on the appropriate insolubilized lectin (Allan et al.. 1972; Hunt and Marchalonis, 1975; Findlay, 1974), usually concanavalin A, which binds to terminal a-D-mannose or a-D-glucose residues of surface glycoproteins. This communication reports a simple and rapid procedure for isolation of purified turkey erythrocyte membranes. These membranes have been analyzed by the techniques discussed above and the major protein and glycoprotein components identified.
t From the Department of Medicine, University of Melbourne, Austin Hospital, Heidelberg, Victoria, 3084, Australia. Receioed December 4 . 1975. A.B.C. is a Postgraduate Medical Research Scholar of the National Heart Foundation of Australia. This work was supported in part by the Petroleum Research Foundation, administered by the American Chemical Society. A preliminary report on part of this work has been presented (Caldwell, 1975). I Abbreviations used are: CAMP,cyclic adenosine 3',5'-rnonophosphate; PAS, periodic acid-Schiff: Con A, concanavalin A.
Experimental Section
Preparation of Purified Turkey Erythrocyte Membranes. Five to twenty milliliters of blood was drawn from the wing vein of either adult male or non-egg-laying female domestic turkels into a heparinized syringe, chilled immediately, and centrifuged for 5 min (600g, 4 " C ) .After removing the plasma and buffy coat, the cells were resuspended in ice-cold 150 m M NaCl buffered with 10 m M Tris-HC1 to pH 7.4. The cells were centrifuged again for 5 min (600g, 4 "C), and the supernatant and top 1-mm layer of cells removed. After two more similar washes, the erythrocytes were hemolyzed in 10 m M potassium phosphate buffer, p H 7.4, containing 4 m M MgS04, using a ratio of 40 ml of hemolyzing buffer to 1 ml of packed cells. In all low ionic strength buffers, Mg2+ was necessary to preserve the integrity of the nuclei. After centrifugation of the hemolysate (25 OOOg, 4 "C, 10 min), a two layered pellet was visible: BIOCHEMISTRY, VOL
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the upper pink pellet contained nucleated erythrocyte ghosts, and the lower small, white pellet contained cells rich i n granular material as viewed under phase contrast microscopy. This lower pellet probably represents protease-rich leukocytes which are not removed even by extensive initial washings, as noted by other workers (Fairbanks et al., 1971). Care must be exercised to prevent contamination of the erythrocyte ghost fraction with these leukocytes since this results in proteolytic degradation of the final membrane product. The supernatant was aspirated and the erythrocyte ghosts were gently resuspended by swirling in the above hemolyzing buffer (20 ml/ml initial packed cells) to which 0.3 M sucrose had been added. The nucleated erythrocyte ghosts were decanted, leaving behind the firmly adherent leukocyte pellet and a proportion of the erythrocyte ghosts. After standing on ice for 15 min, the resuspended nucleated ghosts were centrifuged (25 OOOg. 4 "C. I O min). The resultant homogeneous pellet of creamy to pink nucleated ghosts was resuspended in 50 m V NaCI. I O m M potassium phosphate, pH 7.4 (5 ml/ml initial packed cells). and homogenized with a Polytron PT-IO (Kinematica) at a setting of I O for IO s. Mg2+ was not included in the buffer used in this step because it promotes vesicle formation and unsealed membranes with both sides accessible to substrate were desired. NaCl (50 m M ) was found to be the lowest molaritq salt that protected the nuclear membrane from significant disruption during homogenization i n the absence of Mg". The homogenized ghosts were centrifuged (50 OOOg. 1 5 min, 4 " C ) and the supernatant was removed; the adherent, viscous bottom layer contained nuclei. The upper ti40 layers containing the membranes and the unbroken ghosts were easily resuspended in 1-4 ml of 5 m M potassium phosphate buffer. pH 7.4. containing 2 mM MgS04, and recentrifuged in I-ml glass centrifuge tubes (Pyrex) for I O min ( I 2 5OOg. 4 " C ) . The final pellet was clearly separated into two thick layers: a cream] pellet of nucleated ghosts covercd bq a thick band of white translucent membranes. After aspiration of the supernatant, the membrane layer was readily resuspended by Pasteur pipet in 5 m M potassium phosphate buffer, pH 7.4, 2 m M MgSO4. to a final concentration of approximatelq 1 mg of membrane protein/ml. I n those experiments where adenylate cyclase activity was measured. the same procedure b a s followed except that 1 m M dithiothreitol was included in the homogenization buffer (50 m M NaCI. I O m M potassium phosphate, p H 7.4) and buffer for subsequent resuspensions was changed to 2-5 m M Tris-HCI, pH 7.4, 5 m M MgS04, I m M dithiothreitol. Preparation of Human E r j t h r o q t e Ghosts. This was performed as described by Dodge et al. (1963). Lai.topero_uidase-CataILzt.d Iodination, This was performed as described by Marchalonis et al. (1971). using 5 X IO' washed erythrocytes i n a total volume of 250 p l . For those experiments where was incorporated into purified membranes, the same procedure was followed using a total volume of 50 p l , containing approximatell 25 pg of membrane protein. E n z j m e and Chemical Assuys. Acetylcholinesterase was assaqed as described by Steck and Kant (1974). 5'-Uucleotidase was assayed as described by Avruch and Wallach ( 1 97 I ). Adenylate cyclase was assayed as described by Salomon et al. (1974). The 100-p1 incubation mixture contained 25 m M Tris, pH 7.4, 5 m M MgS04, 1 m M dithiothreitol. 1 m M CAMP, 1 m M L Y - [ ~ ~ P ] A(approximately TP IO6 cpm/ tube), 4.2 mM phosphoenolpyruvate, 2.95 units of pyruvate kinase, and other agents as indicated in the figure legends. Cytochrome oxidase was assayed as described by Schnaitman and Greenawalt ( 1968). Lactate dehydrogenase was assayed
as described by Nielands (19%). Hemoglobin was determined spectrophotometrically using Soret band absorption at 4 10 nm as follows: suitable aliquots of various fractions containing 0. I % sodium dodecyl sulfate and 30-60 pg of DNase I/ml were incubated at room temperature for an hour. Samples were read a t 410 nm and the amount of hemoglobin present was calculated using the known absorption 0.863 ;it 540 nm for a I mg/ml solution of hemoglobin (Williams and Tsay. 1973). Weither sodium dodecyl sulfate nor DNase affected the absorption properties of the hemoglobin. D N A uax assayed a s described by Kissane and Robins ( 1958) as modified b), Waldman and Alm ( 1 970) for determination in the presence of sucrose. Using bovine serum albumin as a standard, protein was assayed as described by Lowry et al. ( 195 1 ) and, in cases where dithiothreitol was present, as modified bq Geiger and Bessman ( 1 972). Af j n i t y Chroniatographj, on C'otzcanacalitz A-Srpharose 4B. bsing the method of Allan et al. ( 1 972), purified membranes from I z r l iodinated cells or membranes "51 iodinated after preparation were solubilized by resuspension in 1 50 m M YaCI, I O m M Tris-HCI, pH 7.4. containing 1% deoxycholate at a ratio of approximately 1 mg of membrane protein/ml of I96 deoxycholate solution. After 30 min at 4 "Cwith frequent vortexing. the mixture was centrifuged for either 30 rnin (100 OOOg, 4 "C) or I5 m i n ( 1 2 500g, 4 "C).The two centrifugation procedures were found to give identical results w i t h respect to percent solubilization of either radioactivitq or protein. The supernatants containing the solubilixd and "!I-iodinated membrane proteins were applied to 1 -ml (packed volume) columns of concanavalin A -Sepharosc 4B which had previousl! been washed with I O nil of 1 % deoxqcholate i n Tris-saline buffer, p H 7.4. I O ml of 5% methyl (Yniannopyrannoside in the 1% deoxycholate. Tris-saline buffer. and then finally 20 nil of 1 % deoxycholate in Tris saline. Following application of the sample. columns were washed with 1% deoxycholate in Tris-saline u n t i l radioactivit) in the 0.25-ml effiuent fractions had returned to background. Bound glycoproteins were then eluted with 5% methyl t r - m a n n o p > ranoside in 1 % deoxycholate, Tris-saline. Fractions containing radioactive peaks of both unbound and bound material were pooled and aliquots subjected to polqacr) laniide gel electrophoresis. Po lj%a c r j la n I ide G' e I E lr c't r o p h o re.pis, T his \+;i x p r:r fo r in ed as described bq Fairbanks et al. (1971). Membrane aliyuots wJere solubilized in a buffer containing I?b sodium dodec! I sulfate, 2.5% 2-mercaptoethanol, 10 m M Tris-HCI. p H 8.0. I m M EDTA, and IO06 sucrose and heated at 100 "C for 3 min. Bromphenol blue was added and the soluble membrane proteins (20-50 pug) were electrophoresed on 5.6% gels ( h X 1 10 mm) containing 1% sodium dodecql sulfate. Gels were fixed and stained with either Coomassie blue or pcriodic acid-Schiff reagent a5 previously described (Fairbanks et ai.. 1971). Stained gels s e r e scanned i n ;I Canalco gel scanner utilizing the appropriate filter for either Cooinassic blue o r PAS stain. Gels containing "51-iodinated proteins Mcre sliccd into 1.25-mm slices using a razor blade stack and counted i n a Pa c kard A u ~ og-a ni n7 a spec t r o in e t e r . 41o I ec u I a r \+e i g h t markers used were: phosphorylase A, 94 000: lactopcrouidaae. 77 500: bovine serum albumin. 68 000: catalase. 60 000; glutamic dehydrogenase, 53 000: ovalbumin 43 000: aldolase. 40 000: light chain, 23 000: and trypsin inhibitor. 14 000. Marerials. Lnless otherwise stated. all substrates. coeii7ymes, nucleotides. enzymes. and other agents Mere obtained from Sigma. Calbiochem. or Boehringer. Phosphoenolpyru\,3te \vas obtained from Fine Chemicals of Australia: propranolol
PROTEINS OF THE TURKEY ERYTHROCYTE MEMBRANE ~~
TABLE I: Recovery
of Markers in Fractionation of Turkey Erythrocytes."
1251 Radioactivity
(loop
Spec Radioact.c (61)
20 60
Resuspension and wash (25 OOOg, I O min) Supernatant Nucleated erythrocyte ghosts Polytron homogenization (50 OOOg, 15 min) Supernatant Nucleated erythrocyte ghosts and membranes Nuclei Resuspension (12 500g, I O min) Supernatant Nucleated erythrocyte ghosts Membranes
Hemoglobin
DNA
( 100)
( 1 9.0)
( 1 00)
Spece Quantity (0.60)
16 419
101 2.2
18.6 4.0
83 6.2
0.68 0.28
0
0
66
36.5
18
269
0.5
4.1
1.1
0.24
21
46.8
2 50
54 592
1.4 0.1
14.0 0.5
2.1 0.4
0.63 0.04
0
0
58
56
3.5 16
582 1237
