Biochemical Responses Induced by Herbicides - American Chemical

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Identification of the Receptor Site for Triazine Herbicides in Chloroplast Thylakoid Membranes 1

KATHERINE Ε. STEINBACK, KLAUS PFISTER and CHARLES J. ARNTZEN Michigan State University, MSU-DOE Plant Research Laboratory, East Lansing, MI 48824

A broad range of inhibitors of photosynthesis act by blocking the same step in electron transport in chloroplast membranes. Inhibition occurs at the level of a protein-bound plastoquinone (B) that functions as the second stable electron acceptor for photosystem II (PS II). Studies based on the use of the proteolytic enzyme, tryp­ sin, indicate that the receptor site for PS II inhibitors is a protein of the structural PS II complex. Using a photoaffinity C-labeled derivative of atrazine, we have identified the specific receptor polypeptide for this inhibitor of PS II function. Analysis of membrane polypep­ tides by polyacrylamide gel electrophoresis and fluorographic techniques have shown that the photoaffinity triazine covalently binds to a polypeptide of 32-34 kilodaltons. We have fur­ ther shown that this polypeptide is surface expos­ ed; trypsin treatment of thylakoid membranes re­ sults in the stepwise alteration of the peptide to a 16 kilodalton species. The site for covalent attachment of the photoaffinity probe is located on the intrinsic 16 kilodalton fragment. Conse­ quently, the binding site appears to be deter­ mined, in part, by the intrinsic hydrophobic do­ main of the 32-34 kilodalton polypeptide. The biogenesis of the 32-34 kilodalton polypeptide is discussed with relation to genetic mechanisms that may be responsible for triazine resistance at the level of the chloroplast membrane. 14

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Current address: University of Wurzburg, Botanical Institute, Wurzburg 87, West Ger­

many. 0097-6156/82/0181-0037$05.00/0 © 1982 American Chemical Society Moreland et al.; Biochemical Responses Induced by Herbicides ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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Approximately h a l f of a l l commercial h e r b i c i d e s act by i n h i b i t i n g photosynthesis by i n t e r a c t i n g with s p e c i f i c s i t e s along the photosynthetic e l e c t r o n t r a n s p o r t c h a i n . A number of d i v e r s e chemicals i n c l u d i n g the ureas, amides, t r i a z i n e s , t r i a z i n o n e s , u r a c i l s , p y r i d a z i n o n e s , q u i n a z o l i n e s , t h i a d i a z o l e s , and c e r t a i n phenols are thought to act s p e c i f i c a l l y at a common i n h i b i t o r y s i t e at the reducing s i d e of photosystem II (PS II) (1, 2 ) . Several l i n e s of evidence i n d i c a t e that t h i s i n h i b i t i o n occurs at the l e v e l of a p r o t e i n bound plastoquinone c a l l e d "B" (3). This e l e c t r o n c a r r i e r acts as the second s t a b l e e l e c t r o n acceptor of PS II (4^ 5 ) . It has been proposed that the common mode of a c t i o n of these chemical c l a s s e s i s v i a h i g h - a f f i n i t y binding to the PS II complex {6). Herbicide binding induces a change in the redox p o t e n t i a l of the quinone c o f a c t o r of B, thereby making the t r a n s f e r of e l e c t r o n s from Q (the f i r s t s t a b l e e l e c t r o n acceptor o f PS II) thermodynamically unfavorable (3, 5 ) . Establishment of s t r u c t u r e - a c t i v i t y r e l a t i o n s ï ï i p s within h e r b i c i d e c l a s s e s has been extremely useful in e l u c i d a t i n g important s t r u c t u r a l aspects of the i n h i b i t o r molecules themselves (_7). U n t i l r e c e n t l y , however, much l e s s emphasis has been d i r e c ted at understanding the biochemical c h a r a c t e r i s t i c s of the h e r b i c i d e receptor s i t e within the c h i o r o p l a s t membrane. Progress in t h i s d i r e c t i o n was i n i t i a t e d when Strotmann and T i s c h e r ( β , 8) introduced techniques f o r monitoring s p e c i f i c binding of h e r b i c i d e s to i s o l a t e d c h i o r o p l a s t t h y l a k o i d membranes. These and other workers (_3, j>, 9) have u t i l i z e d a v a r i e t y of r a d i o l a ­ beled i n h i b i t o r s of PS II f u n c t i o n f o r the c h a r a c t e r i z a t i o n of p r o p e r t i e s of the h e r b i c i d e binding s i t e ; the studies have r e s u l ­ ted in the demonstration of a competition f o r a s i n g l e binding domain per photosynthetic e l e c t r o n transport c h a i n . An understanding of the biochemical c h a r a c t e r i s t i c s of the PS II l o c a l i z e d h e r b i c i d e receptor domain is p a r t i c u l a r l y r e l e ­ vant because of the appearance of t r i a z i n e - r e s i s t a n t weed biotypes in the United S t a t e s , Canada, and Europe (3). I n i t i a l attempts at understanding the mechanism(s) of resisTance d i r e c t e d i n v e s t i ­ gators to evaluate a l t e r a t i o n s in uptake, t r a n s l o c a t i o n , or meta­ bolism of t r i a z i n e s . Only small d i f f e r e n c e s between s u s c e p t i b l e and r e s i s t a n t biotypes were e s t a b l i s h e d , these being i n s u f f i c i e n t to e x p l a i n the mechanism of extreme h e r b i c i d e r e s i s t a n c e . As the primary mechanism of action of the s - t r i a z i n e s i n v o l ­ ves i n h i b i t i o n of PS II e l e c t r o n t r a n s p o r t , a t t e n t i o n was also d i r e c t e d at a n a l y s i s of c h i o r o p l a s t r e a c t i o n s in r e s i s t a n t weed biotypes (10, 11, 12). These studies can be summarized as f o l ­ lows: (a) m ΤΓ1 cases studied to date, there is a m o d i f i c a t i o n in the c h i o r o p l a s t membranes of r e s i s t a n t biotypes that changes the c h a r a c t e r i s t i c s of s - t r i a z i n e b i n d i n g ; (b) t h i s m o d i f i c a t i o n r e s u l t s in a l t e r e d binding c h a r a c t e r i s t i c s of other c l a s s e s of h e r b i c i d e s , ( i . e . , only s l i g h t r e s i s t a n c e to ureas, but increased s e n s i t i v i t y to phenols) (see L3 f o r review), and (c) the a l t e r a ­ t i o n of the h e r b i c i d e receptor in r e s i s t a n t weeds is accompanied

Moreland et al.; Biochemical Responses Induced by Herbicides ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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by a d e t e c t a b l e change in the k i n e t i c c h a r a c t e r i s t i c s of e l e c t r o n t r a n s f e r from Q" to Β in the native membranes even 1n the absence of a l l h e r b i c i d e s (3^ 14, Jj>), which i n d i c a t e s that the apoprotein o f Β may be a l t e r e d i n T h e r e s i s t a n t c h l o r o p l a s t s so that the bound quinone c o f a c t o r e x h i b i t s d i f f e r e n t redox p r o p e r t i e s . In t h i s paper, we have summarized our current understanding of the biochemical nature of the t r i a z i n e binding s i t e within the PS II complex. Studies using the p r o t e o l y t i c enzyme t r y p s i n as a s e l e c t i v e , s u r f a c e - s p e c i f i c m o d i f i e r of membrane polypep­ t i d e s and the use of a p h o t o a f f i n i t y t r i a z i n e have been u t i l i z e d s e p a r a t e l y to i d e n t i f y the t r i a z i n e receptor protein as a 32-34 k i l o d a l t o n (kDal) polypeptide of the PS II complex in peas (Pisum sativum L . ) . The nature of the covalent attachment of the p h o t o a f f i n i t y probe has also enabled us to i d e n t i f y the t r i a z i n e receptor p r o t e i n as a product of c h l o r o p l a s t - d i r e c t e d p r o t e i n s y n t h e s i s ; t h i s implies that the s t r u c t u r a l gene f o r the t r i a z i n e receptor polypeptide i s encoded on c h i o r o p l a s t DNA. This i s in agreement with r e p o r t s , based on c l a s s i c a l genetic a n a l y s i s , that t r i a z i n e r e s i s t a n c e in B r a s s i c a campestris L. (16) i s a maternally i n h e r i t e d t r a i t . Materials

and Methods

Chioroplast Isolation. C h l o r o p l a s t s of peas, spinach ( S p i n a c i a o l e r a c e a L . ) , and biotypes of Amaranthus hybridus L. suscept i b l e or r e s i s t a n t t o _ s - t r i a z i n e s were i s o l a t e d and stroma-free t h y l a k o i d s prepared as p r e v i o u s l y described (17). Intact c h l o r o ­ p l a s t s were obtained from pea leaves f o l l o w i n g the method of B l a i r and E l l i s (18). Trypsin Treatment. Trypsin incubations were c a r r i e d out at room temperature as p r e v i o u s l y described (JL9). Trypsin concen­ t r a t i o n s used are s p e c i f i e d in the t e x t . Photoaffinity Labeling. The p h o t o a f f i n i t y l a b e l i n g of pea thyla1 DCPIP). 14

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Figure 2B. Comparison of data derived from studies of C-atrazine affinity for thylakoid membranes incubated with trypsin with that obtained from electron transport assays (H 0 -> DCPIP) in the presence of 0.25 μΜ atrazine. 14

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p r o t e o l y t i c enzyme). This was followed by a gradual change in a f f i n i t y over the next 15 min of treatment. This i s in c o n t r a s t to the gradual time-dependent decrease in binding s i t e s , implying at l e a s t a two-step a l t e r a t i o n in the p r o t e i n (or p r o t e i n s ) of the PS II complex that c o n s t i t u t e the a t r a z i n e binding s i t e . Mild t r y p s i n treatment has been shown also to a l t e r the a f f i n i t y f o r a number of other chemical f a m i l i e s of PS II d i r e c ted h e r b i c i d e s in a manner s i m i l a r to that of the s - t r i a z i n e s . Trypsin-mediated decreases in i n h i b i t o r y a c t i v i t y are found for u r a c i l (19), urea, pyridazinone ( 1 £ , 28) and t r i a z i n o n e (28) herbicides. In c o n t r a s t , p h e n o l - t y p e T e r b i c i d e s increasecTin i n h i b i t o r y a c t i v i t y f o l l o w i n g b r i e f t r y p s i n treatment {19_ 28), although the trend was reversed over longer treatment perioïïs. The d i s t i n c t l y d i f f e r e n t behavior of the phenol-type h e r b i c i d e s f o l l o w i n g t r y p s i n treatment suggests that d i f f e r e n t d e t e r minants within the PS II protein complex e s t a b l i s h the "domains" that r e g u l a t e the binding p r o p e r t i e s of these i n h i b i t o r s . In s p i t e of the f a c t that phenol-type h e r b i c i d e s w i l l d i s p l a c e bound r a d i o l a b e l e d h e r b i c i d e s such as d i u r o n , these i n h i b i t o r s show noncompetitive i n h i b i t i o n (29, 30). At present, there are three l i n e s of evidence which favor Tfie involvement of two domains within the PS II complex that p a r t i c i p a t e in c r e a t i n g the binding s i t e s f o r these h e r b i c i d e s : (a) i s o l a t e d PS II p a r t i c l e s can be s e l e c t i v e l y depleted of a polypeptide with p a r a l l e l loss of a t r a zine s e n s i t i v i t y , but not dinoseb i n h i b i t i o n a c t i v i t y (33); (b) in r e s i s t a n t weed b i o t y p e s , c h i o r o p l a s t membranes that e x h i b i t extreme t r i a z i n e r e s i s t a n c e have increased s e n s i t i v i t y to the phenol-type h e r b i c i d e s (13); and (c) experiments with azido (photoa f f i n i t y ) d e r i v a t i v e s of phenol and t r i a z i n e h e r b i c i d e s r e s u l t in the covalent l a b e l i n g of d i f f e r e n t PS II polypeptides (20, 31). 9

I d e n t i f i c a t i o n of the T r i a z i n e Binding S i t e . An ultimate goal f o r the biochemical understanding of h e r b i c i d a l e f f e c t s on c h i o r o p l a s t membranes is to i d e n t i f y the s p e c i f i c membrane cons t i t u e n t that serves as h e r b i c i d e r e c e p t o r . For the s - t r i a z i n e s , i d e n t i f i c a t i o n of the s p e c i f i c receptor i s r e q u i r e d for understanding t r i a z i n e r e s i s t a n c e at the molecular l e v e l . Because t r y p s i n m o d i f i c a t i o n of membranes r e s u l t s in p r o t e i n - s p e c i f i c a l t e r a t i o n s of the membrane, the stepwise loss of h e r b i c i d e binding a f f i n i t y and binding s i t e s is a t t r i b u t e d to the stepwise a l t e r a t i o n of a protein that serves as the h e r b i c i d e r e c e p tor. One approach to the i d e n t i f i c a t i o n of h e r b i c i d e receptors has been the a n a l y s i s of peptide a l t e r a t i o n s f o l l o w i n g t r y p s i n treatment by using SDS-PAGE. A n a l y s i s of polypeptide a l t e r a t i o n in t h y l a k o i d membranes has been u n s a t i s f a c t o r y , however, because of the m u l t i p l i c i t y of p r o t e i n changes brought about by t r y p s i n (19). However, c h a r a c t e r i z a t i o n of membranes subfractionated f o l l o w i n g detergent treatment has been somewhat more i n f o r m a t i v e . When PS II enriched s u b f r a c t i o n s are i s o l a t e d from t r y p s i n treated membranes, the number of a l t e r e d polypeptides that could

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correspond to an a l t e r e d receptor p r o t e i n i s narrowed down to four candidates. Among these i s a polypeptide of 32 kDal (19). Highly p u r i f i e d PS II complexes i s o l a t e d by detergent f r a c t i o n a t i o n also contain a polypeptide of 32 kDal (32, 33). Trypsin, treatment of i s o l a t e d PS II complexes r e s u l t s in the degradation of the 32 kDal polypeptide with concomitant l o s s of h e r b i c i d e a c t i v i t y (32, 33). F i n a l l y , s e l e c t i v e removal of the 32 kDal polypeptide from PS II p a r t i c l e s by a d d i t i o n a l detergent t r e a t ments r e s u l t s in loss of both diuron and a t r a z i n e binding (33). These l i n e s of evidence a l l i m p l i c a t e a 32 kDal polypeptide as the receptor f o r t r i a z i n e and urea c l a s s e s of h e r b i c i d e s . The a s s o c i a t i o n of diuron and a t r a z i n e with c h i o r o p l a s t membranes i s v i a a h i g h - a f f i n i t y , but noncovalent b i n d i n g . Attempts at p h y s i c a l i s o l a t i o n of proteins labeled by r a d i o a c t i v e i n h i b i t o r s have f a i l e d because techniques such as detergent f r a c t i o n a t i o n or e l e c t r o p h o r e t i c separation r a p i d l y lead to a new e q u i l i b r i u m and the d i s s o c i a t i o n of the noncovalent r e c e p t o r - i n h i b i t o r complex. One approach that overcomes t h i s d i f f i c u l t y in i d e n t i f y i n g a h e r b i c i d e receptor i s to attach a r a d i o labeled p h o t o a f f i n i t y azido d e r i v a t i v e of the h e r b i c i d e to i t s h i g h - a f f i n i t y receptor p r o t e i n . A c t i v a t i o n of the azido f u n c t i o n of p h o t o a f f i n i t y compounds by UV i r r a d i a t i o n produces a n i t r e n e that i s h i g h l y r e a c t i v e (34). If the compound remains l o c a l i z e d at i t s h i g h - a f f i n i t y s i t e throughout the l i f e t i m e of the d e s t a b i l ized n i t r e n e group, covalent binding w i l l occur at the binding site. A z i d o a t r a z i n e has been shown to i n h i b i t photosynthetic e l e c t r o n t r a n s p o r t at a s i t e i d e n t i c a l to that of a t r a z i n e (20). We have used a z i d o a t r a z i n e as a p h o t o a f f i n i t y probe to ident~T7y the h e r b i c i d e - r e c e p t o r p r o t e i n in c h l o r o p l a s t s of _A. hybridus. In order to demonstrate the s p e c i f i c i t y of binding to a h i g h a f f i n i t y s i t e , membranes from both s u s c e p t i b l e and r e s i s t a n t biotypes were u t i l i z e d . A n a l y s i s of membrane polypeptides from s u s c e p t i b l e and r e s i s t a n t membranes by SDS-PAGE i s shown in Figure 3 (Coomassie blue stained polypeptides are in lanes A ) . No major d i f f e r e n c e s in polypeptide composition or s t a i n i n g i n t e n s i t y between the two samples are apparent. For both c a s e s , membranes were UV i r r a d i a t e d in the presence of a z i d o - ^ C a t r a z i n e p r i o r to e l e c t r o p h o r e s i s . A n a l y s i s of the gel by a f l u o r o g r a p h i c technique showed no detectable bound r a d i o l a b e l a s s o c i a t e d with the membrane sample from r e s i s t a n t c h l o r o p l a s t s . In the s u s c e p t i b l e membranes, however, the r a d i o l a b e l extended from a region corresponding to 34 kDal to that of a stained polypeptide at 32 kDal. We s h a l l present evidence in the f o l l o w i n g d i s c u s s i o n that t h i s pattern of l a b e l i n g i s due to covalent h e r b i c i d e a s s o c i a t i o n to a s i n g l e p r o t e i n which e x i s t s in e i t h e r of two molecular weight s p e c i e s : a developmental precursor form of 34 kDal which is s i z e - p r o c e s s e d to a 32 kDal form. For the remaining d i s c u s s i o n we s h a l l designate t h i s t r i a z i n e h e r b i c i d e receptor as the 32-34 kDal polypeptide.

Moreland et al.; Biochemical Responses Induced by Herbicides ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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Proc. Natl. Acad. Sci.

Figure 3. Polyacrylamide slab gel elec­ trophoresis of thylakoid membrane poly­ peptides from susceptible and resistant biotypes of A. hybridus, stained for pro­ tein (lanes A) and by fluorography (lanes B). Susceptible and resistant membranes were incubated with 0.5 μΜ azido- Catrazine under UV light for 10 min prior to SDS solubilization. The predominant location of the radiolabel, as shown by fluorography, is over the 34- to 32-kDal size class. 14

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One a d d i t i o n a l l i n e of evidence that independently i m p l i c a t e s a 32 kDal polypeptide in t r i a z i n e binding comes from studies on c h l o r o p l a s t s i s o l a t e d from a maize mutant that s p e c i f i c a l l y lacks the s t a i n a b l e 32 kDal polypeptide and the r a p i d l y labeled 34 kDal polypeptide. C h l o r o p l a s t s of t h i s PS I I - l e s s l e t h a l mutant lack binding s i t e s f o r r a d i o a c t i v e a t r a z i n e (35). Step-wise M o d i f i c a t i o n of the Herbicide Binding Polypeptide In V i t r o . Evidence from a t r a z i n e binding studies described in a previous s e c t i o n suggested that trypsin-mediated a l t e r a t i o n s of surface exposed membrane polypeptides r e s u l t e d i n a sequential a l t e r a t i o n of a t r a z i n e binding s i t e s . F i r s t , a rapid alteration of h e r b i c i d e a f f i n i t y was detected followed by a more gradual loss of d e t e c t a b l e binding s i t e s . This stepwise a l t e r a t i o n was f u r t h e r i n v e s t i g a t e d at the l e v e l of polypeptide s t r u c t u r e by using the s e l e c t i v e membrane m o d i f i e r , t r y p s i n , against membranes that had been c o v a l e n t l y tagged with the r a d i o l a b e l e d photoa f f i n i t y t r i a z i n e probe. As shown in Figure 4A, the use of low c o n c e n t r a t i o n s of a z i d o - ^ C - a t r a z i n e r e s u l t e d in l a b e l i n g of polypeptide species at 34 kDal. We i n t e r p r e t these data as i n d i c a t i n g that the 34 kDal form of the 32-34 kDal polypeptide creates the h i g h - a f f i n i t y h e r b i c i d e binding s i t e . In membrane samples treated with 2 yg trypsin/ml f o r 15 min (see legend of Figure 4 f o r d e t a i l s ) , a t r y p s i n concentration shown p r e v i o u s l y to p r i n c i p a l l y bring about changes in a t r a z i n e a f f i n i t y (Figure 2B), the 34 kDal polypeptide l a b e l e d with a z i d o a t r a z i n e was a l t e r ed to a species that comigrated with a stained 32 kDal polypept i d e (Figure 4B). At higher t r y p s i n concentrations (Figures 4C and 4D; 10 and 40 ug t r y p s i n / m l , r e s p e c t i v e l y ) where loss of both e l e c t r o n t r a n s p o r t f u n c t i o n and i n h i b i t o r binding s i t e s were observed p r e v i o u s l y , the polypeptide tagged by a z i d o a t r a z i n e was f u r t h e r degraded to species at 18 and then 16 kDal in a s e q u e n t i a l , stepwise manner. [The experiment of Figure 4 u t i l i z e d membranes which were f i r s t l a b e l e d with a z i d o a t r a z i n e and then subjected to t r y p s i n treatment.] We conclude that the major covalent attachment s i t e f o r a z i d o a t r a z i n e is in a hydrophobic region of the membrane which is i n a c c e s s i b l e to t r y p s i n , thereby leaving an i n t r i n s i c 16 kDal fragment of the t r i a z i n e binding p r o t e i n associated with the membrane f o l l o w i n g t r y p s i n treatment. Further degradation of the 16 kDal polypeptide is not observed a f t e r prolonged t r y p s i n treatment. The data of Figure 4 i n d i c a t e d that a 34 kDal form of the 32-34 kDal polypeptide i s the h i g h - a f f i n i t y binding s i t e f o r triazines. To t e s t whether the t r y p s i n - d e r i v e d , membrane-bound fragments of t h i s p r o t e i n s t i l l bound h e r b i c i d e , a z i d o a t r a z i n e was used against t r y p s i n - t r e a t e d membranes. Figure 5 shows the fluorogram of e l e c t r o p h o r e t i c a l l y separated pea c h i o r o p l a s t polypeptides from c o n t r o l (A and B) and t r y p s i n - t r e a t e d (C and D) membranes that were tagged with a z i d o a t r a z i n e a f t e r the protease m o d i f i c a t i o n . When a z i d o a t r a z i n e was applied against

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Figure 4. Fluorogram of a polyacrylamide gel that shows trypsin sensitivity of the 34-kDal polypeptide of pea chioro­ plast membranes following covalent radiolabeling with azido- C-atrazine. Key: A, control; B, 2 μg trypsin/mL; C, 10 μ-g trypsin/mL; D, 40 pg trypsin/mL for 15 min at room temperature. Proteolysis was stopped by the addition of a 20-fold excess of trypsin inhibitor. Membranes were washed twice in PSNM buffer prior to electrophoresis. 14

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Figure 5. Fluorogram of electrophoretically separated pea chioroplast poly­ peptides from control and trypsin-treated membranes that were tagged with radio­ labeled azidoatrazine. Key: A, control membranes (50 fig Chl/mL) UV-irradi­ ated 10 min in the presence of 2.5 μΜ azido- C-atrazine; B, as in A, but with 25 μΜ azidoatrazine; C, treatment of chloroplasts with 2 μg trypsin/mL for 15 min prior to tagging with 25 μΜ azidoatrazine; D, as in C, but 40 μg tryp­ sin/mL for 15 min prior to tagging with azidoatrazine. 14

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c o n t r o l membranes at a concentration of 2.5 uM (equivalent to an a t r a z i n e concentration r e q u i r e d to i n h i b i t e l e c t r o n t r a n s p o r t by 90%) o n l y the 34 kDal polypeptide species was labeled (Figure 5A). When a 1 0 - f o l d higher concentration is u t i l i z e d (25 y M ) , equivalent to an a t r a z i n e concentration r e q u i r e d to i n h i b i t e l e c t r o n t r a n s p o r t by 90% in t r y p s i n - t r e a t e d membranes) both the 34 and 32 kDal forms of the 32-34 kDal polypeptide are labeled (Figure 5B). Mild t r y p s i n - t r e a t m e n t (2 yg t r y p s i n / m l , 15 min) followed by a z i d o a t r a z i n e tagging using 25 yM a z i d o a t r a z i n e (Figure 5C) r e s u l t e d in tagging of the 32 kDal species ( i n c o n t r a s t to l a b e l ­ ing both the 34 and 32 kDal forms of the polypeptide in c o n t r o l membranes). This supports the hypothesis that the presence of the t r y p s i n - d e r i v e d 32 kDal polypeptide provides both an a f f i n i t y s i t e and a binding s i t e f o r t r i a z i n e s . When thylakoids are tagged f o l l o w i n g high t r y p s i n treatment (40 yg t r y p s i n / m l , 15 min) no label is observed at 32 kDal, nor at the molecular weights of the expected breakdown products of 18 and 16 kDal. These data f u r t h e r suggest that a f f i n i t y and/or binding to the i n t r i n s i c fragments alone can not occur. It should be noted that in a l l cases where a high concentration of a z i d o a t r a z i n e i s u t i l i z e d (25 yM, Figures 5B, C, and D) a d e t e c t a b l e l e v e l of n o n s p e c i f i c binding of the compound to a number of t h y l a k o i d polypeptides i s observed. The most i n t e n s e l y l a b e l e d was a p o l y ­ peptide species of 25 kDal before t r y p s i n treatment or 23 kDal a f t e r protease d i g e s t i o n . This polypeptide was p r e v i o u s l y demon­ s t r a t e d to be the apoprotein of the l i g h t - h a r v e s t i n g c h l o r o p h y l l a/b pigment p r o t e i n which has a surface-exposed segment (23). The T r i a z i n e Receptor P r o t e i n i s a Product of C h l o r o p l a s t Directed P r o t e i η Synthesfs"] T r i a z i n e r e s i s t a n c e has been demon­ s t r a t e d from r e c i p r o c a l c r o s s i n g experiments to be i n h e r i t e d u n i p a r e n t a l l y through the female parent i n J3. campestri s (16). As the r e s i s t a n c e mechanism has been shown to r e s i d e at the l e v e l of the c h i o r o p l a s t membrane, a r o l e f o r the c h i o r o p l a s t genome i n c o n f e r r i n g r e s i s t a n c e is strongly implied (17). It was of i n t e r e s t to determine i f the c h i o r o p l a s t membrane p r o t e i n of 32-34 kDal that binds the p h o t o a f f i n i t y t r i a z i n e , and which appears to be required f o r t r i a z i n e binding in i s o l a t e d PS II p a r t i c l e s , i s a c h i o r o p l a s t gene product. In developing c h l o r o p l a s t s , in p a r a l l e l to the appearance of f u n c t i o n a l a c t i v i ­ t i e s , there i s r a p i d synthesis and accumulation of a major t h y l a ­ koid p r o t e i n of 34 kDal (36). This r a p i d l y synthesized c h i o r o ­ p l a s t p r o t e i n has been shown to be encoded by the c h i o r o p l a s t genome i n 1. mays (37). This s e c t i o n o u t l i n e s experiments that were c a r r i e d out to determine i f the c h i o r o p l a s t polypeptide, which serves as the t r i a z i n e binding s i t e , i s i d e n t i c a l to the chloroplast-encoded p r o t e i n of the same molecular weight. As was shown i n Figure 4, the 34 kDal polypeptide l a b e l e d by a z i d o a t r a z i n e shows a s p e c i f i c stepwise a l t e r a t i o n mediated

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by the p r o t e o l y t i c enzyme t r y p s i n . We i n t e r p r e t that the s p e c i f i c stepwise a l t e r a t i o n o f t h i s polypeptide is regulated by (a) the presence of l y s i n e or a r g i n i n e residues in the primary sequence of t h i s peptide that are s t e r i c a l l y a c c e s s i b l e to the p r o t e o l y t i c probe and (b) the increased a c c e s s i b i l i t y of s p e c i f i c cleavage s i t e s caused by trypsin-mediated a l t e r a t i o n s of other surface-exposed peptides that share a common microenvironment at the membrane surface with the 32-34 kDal polypeptide. The s t e p wise t r y p s i n degradation of the 34 kDal form of the polypeptide to 32, 18,and 16 kDal d e f i n e s a "map" of i t s s t r u c t u r a l i n t e g r a t i o n within the membrane and can be used to i d e n t i f y the same p r o t e i n "tagged" by an independent method. The c h l o r o p l a s t - s y n t h e s i z e d p r o t e i n of the same molecular weight as the t r i a z i n e - b i n d i n g protein i s the most r a p i d l y synt h e s i z e d p r o t e i n in vivo of pea t h y l a k o i d membranes. Thus, i t s t r y p s i n s e n s i t i v i t y can be r e a d i l y evaluated by autoradiographic techniques. Pea seedlings were allowed to take up and i n c o r p o r ate 3 5 s _ e t h i o n i n e f r 4 h; t h i s r e s u l t e d in r a d i o l a b e l i n g of r a p i d l y synthesized p o l y p e p t i d e s . Control and t r y p s i n - t r e a t e d membrane samples were subjected to SDS-PAGE. Following s t a i n i n g for p r o t e i n , polypeptides were analyzed for r a d i o l a b e l i n c o r p o r a t i o n by X-ray fluorography. As shown in Figure 6A, i n c o r p o r a t i o n of 3 5 $ - r a d i o l a b e l was observed f o r two major molecular weight species — the apop r o t e i n of the l i g h t harvesting complex (LHC) at 25 kDal and a polypeptide of 34 kDal. Following t r y p s i n treatment (Figure 6B), the r a d i o l a b e l associated with the LHC polypeptides was a l t e r e d in e l e c t r o p h o r e t i c m o b i l i t y by 2 kDal f o l l o w i n g the expected a l t e r a t i o n f o r the Coomassie-stained peptide species ( 2 3 ) . The r a p i d l y synthesized 34 kDal polypeptide was also s u s c e p t i b l e to trypsin. Corresponding to i t s loss with t r y p s i n treatment, new r a d i o l a b e l e d bands appeared at 32 (Figure 6B), then 18, and 16 kDal (Figures 6C,D) i n an i d e n t i c a l , t r y p s i n - c o n c e n t r a t i o n dependent fashion to that of the p h o t o a f f i n i t y tagged 34 kDal p o l y peptide (see Figure 4 ) . From the i d e n t i c a l t r y p s i n s e n s i t i v i t y of the p h o t o a f f i n i t y tagged polypeptide and the r a p i d l y synthes i z e d c h i o r o p l a s t p r o t e i n of the same molecular weight, we conclude that the two polypeptides are one and the same. m

0

Evidence that the T r i a z i n e Binding Protein i s Present in r i a z i n e Resistant Weed B i o t y p e s . The u t i l i z a t i o n of a photoa f f i n i t y labeled t r i a z i n e h e r b i c i d e has been i n v a l u a b l e in the d e f i n i t i v e i d e n t i f i c a t i o n of one s p e c i f i c polypeptide of the PS II complex as the t r i a z i n e binding s i t e . The absence of covalent l a b e l i n g in r e s i s t a n t membranes suggests that e i t h e r the polypeptide i s missing from the membrane or that i t i s present, but g e n e t i c a l l y a l t e r e d , r e s u l t i n g in an a l t e r a t i o n in i t s primary s t r u c t u r e , and p o s s i b l y changing i t s conformation in the membranes. In s u s c e p t i b l e membranes, t r i a z i n e and urea h e r b i c i d e s com-

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Figure 6. Fluorogram of polyacrylamide gel showing trypsin sensitivity of the 34-kDal polypeptide of pea chioro­ plast membranes following in vivo incor­ poration of S-methionine in whole leaves. Key: A, control; B, 2 ^g trypsin/ mL; C, 10 ng trypsin/mL; D, 40 μg trypsin/mL. Treatments were as described in Figure 4. 35

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pete f o r the same binding region as determined by d i r e c t compet i t i o n s t u d i e s (9). In r e s i s t a n t membranes, whereas t r i a z i n e s e n s i t i v i t y i s e x t e n s i v e l y d i m i n i s h e d , the a b i l i t y of diuron to i n h i b i t e l e c t r o n t r a n s p o r t i s not a l t e r e d s i g n i f i c a n t l y (3., 9., 13). This i n d i c a t e s an a l t e r a t i o n in the t r i a z i n e , but not the urea a f f i n i t y s i t e and f u r t h e r suggests that a common binding s i t e , i . e . , p r o t e i n , i s present in r e s i s t a n t membranes, but possesses an a l t e r e d a f f i n i t y f o r the t r i a z i n e h e r b i c i d e s . It was of i n t e r e s t to determine i f the polypeptide r e s p o n s i b l e for h e r b i c i d e s e n s i t i v i t y i n c h l o r o p l a s t s of normal, t r i a z i n e - s u s c e p t i b l e plants was also present, i . e . , s y n t h e s i z e d , in the r e s i s t a n t biotypes. Using the techniques f o r 3 5 $ - e t h i o n i n e i n c o r p o r a t i o n j j i v j v o described f o r peas, i n c o r p o r a t i o n of ^ S - r a d i o label i n t o t h y l a k o i d polypeptides of s u s c e p t i b l e and r e s i s t a n t biotypes of A. hybridus was i n v e s t i g a t e d . The data shown in Figure 7 demonstrate that the 34 kDal polypeptide i s synthesized in both h e r b i c i d e - s u s c e p t i b l e and r e s i s t a n t p l a n t s . Furthermore, the polypeptide in r e s i s t a n t membranes shows an i d e n t i c a l s e n s i t i v i t y to t r y p s i n - t r e a t m e n t as the 34 kDal polypeptide of the s u s c e p t i b l e membranes. Whereas i t is evident that the polypept i d e i s synthesized and present in both b i o t y p e s , there i s no apparent s i z e d i f f e r e n c e or change in membrane o r i e n t a t i o n , as r e f l e c t e d by t r y p s i n s e n s i t i v i t y , that can account for the extreme d i f f e r e n c e s in t r i a z i n e a f f i n i t i e s . The t r i a z i n e - r e s i s t a n c e mechanism, t h e r e f o r e , probably r e s i d e s in an a l t e r e d primary s t r u c t u r e of t h i s p r o t e i n . m

5

Acknowledgements: This research was supported, in p a r t , by a Grant from the United States - I s r a e l B i n a t i o n a l A g r i c u l t u r a l Research and Development Fund (BARD), DOE Contract DE-AC02-7ER01338 to Michigan State U n i v e r s i t y , and a Wellesley College F a c u l t y Aid Grant to K. Steinback. We also thank Dr. Gary Gardner of S h e l l A g r i c u l t u r a l Chemicals f o r his c o l l a b o r a t i o n and c o n s u l t a t i o n s in experiments using a z i d o - a t r a z i n e .

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Figure 7. Fluorogram of a polyacrylamide gel showing trypsin sensitivity of the 34-kDal polypeptide of chioroplast thylakoid membranes isolated from susceptible (S) and resistant (R) biotypes of A. hybridus following in vivo incorporation of S-methionine in whole leaves. Isolated thylakoid membranes were treated with A, no trypsin; B, 2 fig trypsin/mL; C, 20 μg trypsin/mL for 15 min as described for Figure 4. 35

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Renger, G. Z. Naturforsch. 1979, 34c, 1010-14. Trebst, A. Z. Naturforsch. 1979, 34c, 986-91. Böger, P.; Kunert, K. J. Z. Naturforsch. 1979, 34c, 1015-25. Reimer, S.; Link, K.; Trebst, A. Z. Naturforsch. 1979, 34c, 419-26. Oettmeier, W.; Masson, K. Pestic. Biochem. Physiol. 1980, 14, 86-97. Oettmeier, W.; Masson, K.; Johanningmeier, U. FEBS Lett. 1980, 118, 267-70. Croze, E.; Kelly, M.; Horton, P. FEBS Lett. 1979, 103, 22-6. Mullet, J. E.; Arntzen, C. J. Biochim. Biophys. Acta 1981, 635, 236-48. Bayley, H.; Knowles, J. R. "Methods in Enzymology", Vol. XLVI; Jackoby, W. B.; Wilchek, M., Eds.; Academic Press: New York, 1977; 69-114. Leto, K.; Keresztes, Α.; Arntzen, C. J. Plant Physiol. 1981, (in press). Grebanier, A. E.; Steinback, Κ. E.; Bogorad, L. Plant Physiol. 1979, 63, 436-9. Bedbrook, J. R.; Link, G.; Coen, D. M.; Bogorad, L.; Rich, A. Proc. Natl. Acad. Sci., U.S.A. 1978, 75, 3060-4.

RECEIVED

August 14, 1981.

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