No abstract

ABSTRACT2-Azido-4 -ethylamino-6 -isopropylamino-s-triazine (azido-atrazine) inhibits photosynthetic electron transport at a site identical to that affected by atrazine (2-chloro4-ethylamino-6-isopropylamino-s-triazine). The latter is a well-characterized inhibitor of photosystem II reactions. Azido-atrazine was used as a photoaffinity label to identify the herbicide receptor protein; UV irradiation of chloroplast thylakoids in the presence of azido ['4C] Many commercial herbicides inhibit photosynthetic electron transport by interrupting electron flow at the reducing side of photosystem II (PS II) (1, 2). There are several lines of evidence which indicate that this inhibition occurs at the level of a protein-bound plastoquinone called "B" (3). This electron carrier acts as the second stable electron acceptor of PS II (4, 5). It has been proposed that the mode of action of compounds such as atrazine or diuron is via high-affinity binding to the PS II complex (6). Herbicide binding induces a change in the redox potential of the quinone cofactor of B, thus making the transfer of electrons from the primary acceptor (Q) thermodynamically unfavorable (3,5). In addition, binding may decrease kinetic interactions of Q and B.Evidence that PS II inhibitors interact with a polypeptide of the PS II complex, possibly the apoprotein of B, comes from studies involving the enzyme trypsin (7). Proteolytic digestion of surface-exposed membrane polypeptides results in a destruction of inhibitor binding sites with a concomitant inactivation of the secondary acceptor, B (8). Attempts have.been made to correlate trypsin-mediated changes in polypeptides of the chloroplast membrane or detergent-solubilized PS II particles with changes in inhibitor binding properties (9). To date, however, the polypeptide(s) that determines the inhibitor binding site has not been identified.A variety of radiolabeled inhibitors of PS II have been used for the characterization of the properties of the herbicide-binding site; these studies have resulted in the demonstration of a single binding site per electron transport chain (3, 10). In addition, the affinity of various inhibitors for this site has been determined (6, 8). The association of PS II inhibitors with their binding site is noncovalent. For this reason, attempts at physical isolation of proteins labeled by a radioactive inhibitor have failed because detergent fractionation or electrophoretic separation rapidly leads to a new equilibrium and dissociation of the acceptor-inhibitor complex. The approach we have used to overcome this difficulty in identification of the herbicide receptor is to attach a radiolabeled photoaffinity azido derivative of atrazine to its high-affinity receptor polypeptide in a covalent manner. It is well established that activation of the azido function of photoaffinity-labeled compounds by UV irradiation produces a nitrene that is highly reactive (11). In preliminary investigations this was found to covalently link the azido-atrazine to chloroplast membranes (12, 13). MA...

Incubation of isolated chloroplast thylakoid membranes with [y-32PJATP results in phosphorylation of surface-exposed segments of several membrane proteins. The incorporation of 32p is light dependent, is blocked by 3(3,4-dichlorophenyl-1,1-dimethylurea (diuron, an inhibitor of electron transport), but is insensitive to uncouplers of photophosphorylation. Polypeptides of the light-harvesting chlorophyll a/ b-protein complex are the major phosphorylated membrane proteins. Addition of ATP to isolated chloroplast thylakoid membranes at 20'C results in a time-dependent reduction of chlorophyll fluorescence emission; this is blocked by diuron but not by nigericin. ADP could not substitute for ATP. Chlorophyll fluorescence induction transients showed a decrease in the variable component after incubation of the membranes with ATP. Chlorophyll fluorescence at 77 K of phosphorylated thylakoid membranes showed an increase in long-wavelength emission compared with dephosphorylated controls. We conclude that a membrane-bound protein kinase can phosphorylate surface-exposed segments of the light-harvesting pigmentprotein complex, altering the properties of its interaction with the two photosystems such that the distribution of absorbed excitation energy increasingly favors photosystem I. The photosynthetic membranes (thylakoid membranes) of plant (1) and green algal (2) chloroplasts contain several polypeptides that are reversibly phosphorylated on surface-exposed threonyl residues (3). The protein kinase that phosphorylates the polypeptides is membrane bound, is dependent on Mg2+, and is activated by light (4). Dephosphorylation is due to a thylakoid-bound phosphatase that is stimulated by Mg2+ but is not dependent upon illumination (3). The favored substrates of both enzymes are two polypeptides (24,000 and 26,000 daltons) that are constituents of the light-harvesting chlorophyll-protein complex (LHC) (5, 6). Together these polypeptides account for about 30% of the total intrinsic thylakoid protein. Their function is to organize about half the chlorophyll (Chl) a and most of the Chl b of the membrane into an antenna complex that absorbs photons and subsequently directs excitation energy to the core complexes of photosystems I and II (PS I and II) (7-9).Photosynthetic electron transport in chloroplasts is mediated by two light reactions acting in series; maximal efficiency of noncyclic electron transport requires a balanced distribution of absorbed excitation energy to the PS II and PS I reaction centers. It is generally accepted that an in vivo regulatory mechanism controls this distribution under varying environmental conditions (10). This regulatory mechanism requires the presence, in chloroplast membranes, of the LHC serving PS II (11). In this paper we present results that show that a physiological role of membrane protein phosphorylation in chloroplasts is to regulate the distribution of absorbed excitation energy between PS I and II. METHODSPlants. Seedlings of dwarf pea (Pisum sativum L. var. Progress 9) wer...

The triazine herbicides inhibit photosynthesis by blocking electron transport at the second stable electron acceptor of photosystem H. This electron transport component of chloroplast thylakoid membranes is a protein-plastoquinone complex termed "B." The polypeptide that is believed to be a component of the B complex has recently been identified as a 32-to 34-kllodalton polypeptide by using a photoaffinity labeling probe, azido-[14Clatrazine. A 34-kilodalton polypeptide ofpea chloroplasts rapidly incorporates [3S]methionine in vivo and is also a rapidly labeled product of chloroplast-directed protein synthesis. roplasts results in-identical, sequential alterations of the 34-kldodalton polypeptide to species of 32, then 18 and 16 kldodaltons. From the identical pattern ofsusceptibility to trypsin we conclude that the rapidly synthesized 34-kilodalton polypeptide that is a product ofchloroplast-directed protein synthesis is identical to the triazine herbicide-binding protein ofphotosystem II. Chloroplasts of both triazine-susceptible and triazine-resistant biotypes of Amaranthus hybridus synthesize the 34-kilodalton polypeptide, but that of the resistant biotype does not bind the herbicide. When dark-grown seedlings are transferred to light, a sequence of developmental processes leads to the formation of green, photosynthetically competent chloroplasts (1). Rapid accumulation of a major thylakoid protein of 32 kilodaltons (kDal) parallels the appearance offunctional activity (2). In Zea mays, this 32-kDal protein is structurally related to, and is presumably derived from, a 34.5-kDal polypeptide that is the major membrane-bound product of protein synthesis by isolated chloroplasts (3). It has been shown to be encoded by a chloroplast gene in Z. mays (4). A number ofchloroplast-encoded proteins appear to be synthesized in etioplasts of dark-grown maize seedlings (2), but the transcription of the chloroplast gene coding for this polypeptide is light dependent in developing plastids and has thus been described as a "photogene" (5, 6). Until recently, no function had been determined for the photogene 32 product; it has been identified now as a determinant of the electron transport function of a bound plastoquinone molecule in the photosystem II (PS II) complex (7). Its photoregulated synthesis (2, 8), apparent rapid turnover, and continued synthesis throughout all stages ofleafdevelopment (2, 3, 9) suggest a regulatory role for the protein product in PS II function. Purified PS II complexes isolated by detergent fractionation techniques contain a protein component of 32 kDal (10, 11). Certain mutants ofZ. nays lacking PSII function are deficient in a 32-kDal membrane polypeptide (12). Trypsin treatment of thylakoid membranes (13) or of isolated PS II complexes (10) results in the degradation of the 32-kDal polypeptide with concomitant loss of PS II activity. These lines of evidence also suggest that the 32-kDal polypeptide plays an integral role in PS II electron transport.A broad range of inhibitors of photo...

No abstract

No abstract

We have compared chloroplast lamellae isolated from a chlorophyll-bless mutant and wild type barley (Hordeum vulgare). The results demonstrate that: (a) one of the two major polypeptides comprising the lightharvesting complex (LHC) is present in the chlorophyll-b-less mutant; (b) higher cation concentrations are required to maintain grana stacks in the mutant; and (c) cation effects on excitation energy distribution are present in the chlorophyll-b-less mutant but are reduced in amount and are 'dependent on higher concentrations of cations.We interpret these data to support the concept that the LHC mediates cation-induced grana stacking and cation regulation of excitation energy distribution between photosystems I and II in chioroplast lamellae. A partial LHC complement in the mutant alters the quantitative cation requirement for both phenomena, but not the over-all qualitative response.The involvement of cations in the regulation of grana formation and in mediating excitation energy distribution between the two photosystems has been extensively examined in recent years (7,9,24). In greening systems, the onset of cation control of energy distribution and the formation of grana stacks appear concomitantly with the synthesis of LHC3 (4, 6, 15). A correlation between the levels of LHC in chloroplast membranes, cation regulation of grana formation, and cation mediation of excitation energy distribution has been shown for almost all higher plant systems studied (1,2,8,29). This has led to the concept that the light-harvesting complex is the primary membrane constituent which is responsible for cation-mediated changes in these chloroplast membrane processes (1, 7).The one exception to the above mentioned rule comes from studies of a chlorina mutant of barley which lacks Chl b. This mutant has been shown to be missing the pigmented light-harvesting Chl a/b protein when examined by SDS-polyacrylamide gel electrophoresis (3,16,18,21,22,28). When examined by electron microscopy this mutant showed either partial or extensive grana stacking (17,22). In addition, cations have an effect on the fluorescence yield of isolated chloroplasts from this barley mutant (10), thus indicating cation-controlled energy distribution processes. Since these data pertaining to the barley mutant tend to invalidate the hypothesis that LHC plays a pivotal role in cationmediated structure-function events, we have reexamined the isolated chloroplasts of the Chl-b-less barley mutant. We will dem-

(8,12,14,25) on the basis of their sizes. We have employed a discontinuous buffer system, which provides stacking of sample proteins, and a polyacrylamide gradient in the running gel to improve resolution of proteins over a wide range of mol wt further.Fluorography of the SDS-polyacrylamide gels was used to determine the number and mol wt of the membrane-associated products synthesized during incubation ofchloroplasts with radioactive amino acids. It is more difficult to quantitate amounts of radioactivity with fluorography than by slicing the gels and counting the radioactivity in each slice. However, fluorography offers the advantages of higher resolution of minor bands and greater precision in aligning peaks of radioactivity with stained proteins on the same gel, making it the most useful technique available for the kinds of comparisons we wished to make. MATERIALS AND METHODSPlant Material. Seeds of Zea mays (FR9CMS x FR37, Illinois Foundation Seeds) were soaked for 20 h, then grown in Vermiculite, either in a dark room or in a greenhouse. Plants kept in the dark room were exposed only to brief periods of green light during watering and handling. They were used 7 to 9 days after planting. For greening of plants, the trays were removed to a room with full lighting, i.e. about 90 to 120 ft-c and room temperature.Chloroplast