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Y Zick

Publications and source records attributed to Y Zick.

At least 19 recordsLinked to original sources
PubMed1992

The insulinomimetic agents H2O2 and vanadate stimulate tyrosine phosphorylation of potential target proteins for the insulin receptor kinase in intact cells.

H2O2 and vanadate are known insulinomimetic agents. Together they induce insulin's bioeffects with a potency which exceeds that seen with insulin, vanadate or H2O2 alone. We have previously shown that a combination of H2O2 and vanadate, when added to intact cells, rapidly stimulates protein tyrosine phosphorylation, owing to the inhibitory effects of these agents on intracellular protein tyrosine phosphatases (PTPases). Employing Western blotting with anti-phosphotyrosine antibodies, we have now identified in Chinese-hamster ovary (CHO) cells transfected with a wild-type insulin-receptor gene (CHO.T cells) several proteins (e.g. pp180, 125, 100, 60 and 52) whose phosphotyrosine content is rapidly increased upon treatment of the cells with a combination of insulin and 3 mM-H2O2. Tyrosine phosphorylation of these and additional proteins was further potentiated when 100 microM-sodium orthovanadate was added together with H2O2. The effects of insulin, insulin/H2O2, and H2O2/vanadate on tyrosine phosphorylation were markedly decreased in CHO cells transfected with an insulin-receptor gene where the twin tyrosines 1162 and 1163 were replaced with phenylalanine (CHO.YF-3 cells). Similarly, most of these proteins failed to undergo enhanced tyrosine phosphorylation in parental CHO cells incubated in the presence of insulin or the insulinomimetic agents. Our findings suggest that inhibition of PTPase activity by H2O2/vanadate augments the autophosphorylation of tyrosines 1162 and 1163 of the insulin receptor kinase, leading to its activation in an insulin-independent manner. As a result, tyrosine phosphorylation of potential targets for this enzyme takes place. Failure of H2O2/vanadate to induce phosphorylation of these proteins in receptor mutants lacking these twin tyrosine residues supports this hypothesis.

Animals↗
PubMed1992

Selective inhibition of protein tyrosine phosphatase activities by H2O2 and vanadate in vitro.

Acute (10-30 min) treatment of intact rat hepatoma (Fao) cells with H2O2, inhibits in vivo protein tyrosine phosphatase activity. Vanadate markedly potentiates this effect although it has only trivial effects of its own. Here we show that H2O2 inhibits a protein tyrosine phosphatase activity, but not a p-nitro phenyl phosphate hydrolysing activity, in cytosolic extracts of these cells. This effect is completely reversed by 10 mM dithiothreitol. Other oxidants have similar inhibitory effects. Vanadate inhibits the protein tyrosine phosphatase activity in vitro, and its effects are additive with those of H2O2. These findings suggest that H2O2 and vanadate interact with the protein tyrosine phosphatases at two independent sites. They also suggest that in intact cells H2O2 has a direct inhibitory effect on protein tyrosine phosphatase activity and an indirect effect of facilitating the entry of vanadate.

Animals↗
PubMed1992

Insulin and insulinomimetic agents induce activation of phosphatidylinositol 3'-kinase upon its association with pp185 (IRS-1) in intact rat livers.

The major cytosolic substrate of the insulin receptor is a 185-kDa phosphoprotein (IRS-1) that contains multiple putative attachment sites for the p85 alpha regulatory subunit of phosphatidylinositol 3'-kinase (PI3K). To examine the possible interaction of pp185 with p85 alpha in vivo, we injected insulin or insulinomimetic agents (a combination of H2O2 and vanadate (H/V)) into the portal vein of anesthetized rats. IN this model system, H/V treatment and, to a lesser extent, injection of insulin resulted in rapid and sustained tyrosine phosphorylation of multiple cellular proteins, including pp185/IRS-1. The latter was found to undergo specific association with the p85 alpha regulatory subunit of PI3K but not with two other proteins that contain src homology domains. As p85 alpha was not detectably phosphorylated on tyrosine residues and did not appear to interact directly with the insulin receptor, we conclude that tyrosine phosphorylation of pp185 promotes its association with p85 alpha and the catalytic subunit of PI3K. The recruitment of the holoenzyme may also involve its enzymatic activation and thus constitute an important step in the transduction of insulin signals.

Animals↗
PubMed1992

The effect of tyrosine-specific protein phosphorylation on the assembly of adherens-type junctions.

Adherens-type junctions (AJs) are major subcellular targets for tyrosine specific protein phosphorylation [Volberg et al. (1991) Cell Regul., 2, 105-120]. Here we report on the apparent effect of such phosphorylation events on the assembly and integrity of AJs. We show that incubation of MDCK cells with potent inhibitors of tyrosine-specific phosphatases (PTP), namely H2O2 and vanadate, leads to a dramatic increase in AJ-associated phosphotyrosine which was apparent already within 2-5 min of treatment and progressed upon further incubation. Examination of H2O2 vanadate treated cells at later time points indicated that intercellular AJs rapidly deteriorated, concomitantly with a marked increase in the number and size of vinculin and actin containing focal contacts. In parallel, major changes were observed in cell structure and topology, as revealed by electron microscopy. These were manifested by rapid rounding-up of the cells followed by reorganization of the cell monolayer. Other intercellular junctions, including desmosomes and tight junctions, visualized by staining with desmoplakin and ZO-I antibodies, were not significantly affected. To verify that modulation of AJs was indeed related to tyrosine phosphorylation, we have carried out reciprocal experiments in which Rovs Sarcoma virus (RSV) transformed chick lens cells, expressing high levels of pp60src kinase, were treated with inhibitors of tyrosine kinases, (tyrphostins). We show that following such treatment, intercellular AJs which were deteriorated in the transformed cells, were reformed. Based on these observations, we propose that specific tyrosine phosphorylation of AJ components is involved in the downregulation of these cellular contacts.

Actins↗
PubMed1992

Defects of insulin's signal transduction in old rat livers.

Aging is associated with a postbinding defect in insulin action, leading to increased glucose intolerance and occasional diabetes. To determine whether defects in insulin receptor kinase (IRK) activity or in the phosphorylation of its physiological substrates underlie this age-related phenomenon, young (2-3 months old) and old (24-27 months old) Wistar rats were studied. When assayed in vitro, the hepatic IRK activities of noninjected old and young rats were comparable. Thirty seconds after the injection of insulin, the hepatic IRK activity of young rats increased 7- to 10-fold in a dose-dependent manner, with maximal effects obtained in rats injected with 20 mg insulin. By contrast, old animals exhibited impaired in vivo activation, with a mean 50% reduction in maximal IRK activity. When the rats were grouped into animals with mild (20%), moderate (50%), and severe (80%) reductions in maximal IRK activity, it was found that the mild and moderate defects could be reversed once the receptors were subjected to extensive autophosphorylation in vitro. The severe form of the defect was essentially irreversible and could not be corrected by phosphorylation in vitro. Immunoblotting with anti P-Tyr antibodies revealed that the reduced IRK activity in the old animals correlated with reduced intrahepatic tyrosine phosphorylation of the beta-subunit of the insulin receptor and pp180, a putative substrate of IRK. We, therefore, conclude that glucose intolerance in aging could be attributed at least in part to acquired defects in the in vivo activation of the hepatic IRK, which results in reduced phosphorylation of its putative substrate pp180.

Aging↗
PubMed1991

Polylysine increases the number of insulin binding sites in soluble insulin receptor preparations.

The effects of cationic polyamino acids on insulin binding to soluble insulin receptor preparations were studied. Incubation of partially or fully purified receptor preparations with polylysine (pLys) increased by several-fold the amount of [125I]insulin that remained associated with the receptor, as determined both by precipitation of receptor-insulin complexes by polyethylene glycol or by separation of the complexes from the free hormone by gel filtration. This elevation in the amount of bound insulin resulted from increased number of insulin binding sites, and could not be attributed to an increased affinity of the receptors to insulin. In fact, pLys reduced 2-3-fold the affinity of insulin binding to its receptor as determined by equilibrium binding studies, and by monitoring the rate of exchange of bound [125I]insulin with unlabeled hormone. pLys induced specific interactions between insulin and its native receptor since other basic compounds such as histone, spermidine, polymixin B, compound 48/80, lysine, and arginine failed to reproduce its effects. pLys did not interact with the free ligand, nor did it promote interactions between insulin and denatured receptor forms. Furthermore, pLys did not induce binding of insulin to other proteins present in the partially purified receptor preparations. The effects of pLys were time and dose-dependent and were proportional to the pLys chain length. The longer the chain, the greater was the effect. Enhanced insulin binding and receptor beta-subunit autophosphorylation (in the presence of insulin) exhibited a similar dependency on the chain length of pLys. pLys effects on insulin binding were associated with formation of large protein aggregates that remained trapped at the top of Sephacryl S-300 columns. These aggregates contained substantial amounts of receptor-insulin complexes. Our results suggest that pLys induces formation of receptor clusters that create de novo insulin binding sites among adjacent receptor tetramers. Alternatively, formation of receptor aggregates might facilitate insulin binding to a soluble receptor subfraction that otherwise fails to bind the hormone.

Animals↗
PubMed1991

pp75: A novel tyrosine-phosphorylated protein that heralds differentiation of HL-60 cells.

The human promyeloid cell line HL-60 differentiates toward monocytes or granulocytes when treated with 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) or dibutyryl cAMP, respectively. When nondifferentiated cells were incubated for 20 min with 2 mM H2O2 and 0.1 mM sodium orthovanadate to inhibit their protein-tyrosine-phosphatase activity (Heffetz, D., Bushkin, I., Dror, R., and Zick, Y. (1990) J. Biol. Chem. 265, 2896-2902), we found marked tyrosine phosphorylation of a single major protein of 53 kDa. Induction of differentiation of HL-60 cells was accompanied by the appearance of an additional major cytosolic tyrosine-phosphorylated protein of 75 kDa (pp75). In dibutyryl cAMP-treated cells, tyrosine phosphorylation of pp75 peaked after 24 h and then declined rapidly. In 1,25(OH)2D3-treated cells, increased tyrosine phosphorylation was detected as early as 2 h and peaked after 3 days, whereas the presence of differentiated phenotypes, assessed by the capacity of the cells to reduce nitro blue tetrazolium, was detected no earlier than 24 h. Doses of 1,25(OH)2D3 as low as 1 nM induced the appearance of pp75 at a stage where almost no differentiation measured by nitro blue tetrazolium reduction was detected. Phosphorylation of pp75 was not stimulated by adriamycin, which induced growth arrest without initiation of differentiation. pp75 could also be detected in U-937, a monocytic cell line that is more advanced in its differentiation state, and also in terminally differentiated circulating human monocytes treated with H2O2/vanadate. pp75 underwent in vitro tyrosine phosphorylation in cytosolic extracts derived from 1,25(OH)2D3-induced HL-60 cells, but not in extracts derived from uninduced cells. Our results raise the possibility that tyrosine phosphorylation of pp75 may be a common early event that heralds the differentiation of HL-60 cells into both the monocytic and granulocytic pathways.

Blotting, Western↗
PubMed1991

Modulation of intercellular adherens-type junctions and tyrosine phosphorylation of their components in RSV-transformed cultured chick lens cells.

Transformation of cultured chick lens epithelial cells with a temperature-sensitive mutant of Rous sarcoma virus (tsRSV) leads to radical changes in cell shape and interactions. When cultured at the restrictive temperature (42 degrees C), the transformed cells largely retained epithelial morphology and intercellular adherens junctions (AJ), whereas on switch to the permissive temperature (37 degrees C) they rapidly became fibroblastoid, their AJ deteriorated, and cell adhesion molecules (A-CAM) (N-cadherin) largely disappeared from intercellular contact sites. The microfilament system that was primarily associated with these junctions was markedly rearranged on shift to 37 degrees C and remained associated mainly with cell-substrate focal contacts. These apparent changes in intercellular AJ were not accompanied by significant alterations in the cellular content of several junction-associated molecules, including A-CAM, vinculin, and talin. Immunolabeling with phosphotyrosine-specific antibodies indicated that both cell-substrate and intercellular AJ were the major cellular targets for the pp60v-src tyrosine-specific protein kinase. It was further shown that intercellular AJ components serve as substrates to tyrosine kinases also in nontransformed lens cells, because the addition of a combination of vanadate and H2O2--which are potent inhibitors of protein tyrosine phosphatases--leads to a remarkable accumulation of immunoreactive phosphotyrosine-containing proteins in these junctions. This finding suggests that intercellular junctions are major sites of action of protein tyrosine kinases and that protein tyrosine phosphatases play a major role in the regulation of phosphotyrosine levels in AJ of both normal and RSV-transformed cells.

Animals↗
PubMed1990

Basic polycations activate the insulin receptor kinase and a tightly associated serine kinase.

The effects of cationic polyamino acids on phosphorylation of the insulin and insulin-like growth factor 1 receptor kinases were studied and the following observations were made. (a) Polylysine stimulated both tyrosine and serine phosphorylation of the insulin receptor and of additional proteins present in lectin-purified membrane preparations from rat liver. (b) Polylysine synergized with insulin to enhance phosphorylation of the insulin receptor and of additional proteins (pp40 and pp110). (c) Polylysine effects were more pronounced upon increasing the polylysine chain length. (d) The effect of polylysine was biphasic with an optimum at 100 micrograms/ml. (e) Polylysine was found ineffective in stimulating the phosphorylation of immobilized insulin receptors. Taken together, these findings support the notion that the action of polylysine involves conformational changes and presumably aggregation of soluble receptors. The same effects of polylysine were obtained with highly purified insulin receptor preparations. Under these conditions polylysine enhanced both serine and tyrosine phosphorylation of the insulin receptor, suggesting that polylysine stimulates the activity of the insulin receptor kinase, and of a serine kinase that is tightly associated with the insulin receptor.

Animals↗
PubMed1990

A combination of H2O2 and vanadate concomitantly stimulates protein tyrosine phosphorylation and polyphosphoinositide breakdown in different cell lines.

Treatment of four cell lines [rat hepatoma (Fao), murine muscle (BC3H-1), Chinese hamster ovary (CHO), and rat basophilic leukemia (RBL)] with a combination of 3 mM H2O2 and 1 mM sodium orthovanadate markedly stimulates protein tyrosine phosphorylation, which is accompanied by a dramatic increase (5-15-fold) in inositol phosphate (InsP) formation. H2O2/vanadate stimulate best formation of inositol triphosphate while their effects on the mono and di derivatives are more moderate. In the presence of 3 mM H2O2, both protein tyrosine phosphorylation and InsP formation are highly correlated and manifest an identical dose-response relationship for vanadate. Half-maximal and maximal effects are obtained at 30 and 100 microM, respectively. This stimulatory effect of H2O2/vanadate is not mimicked by other oxidants such as spermine, spermidine, KMnO4, and vitamin K3. In RBL cells, the kinetics of inositol triphosphate formation correlate with tyrosine phosphorylation of a 67-kDa protein, while tyrosine phosphorylation of a 55-kDa protein is closely correlated with both inositol monophosphate formation and serotonin secretion from these cells. Taken together, these results suggest a causal relationship between tyrosine phosphorylation triggered in a nonhormonal manner and polyphosphoinositide breakdown. Furthermore, these results implicate protein tyrosine phosphorylation in playing a role in the stimulus-secretion coupling in RBL cells.

Animals↗
PubMed1990

Alterations in insulin receptor kinase activity during differentiation of HL-60 cells.

Differentiation of the human promyelocytic leukemia cell line HL-60 into monocytes or macrophages is associated with increased expression of cell surface insulin receptors, while differentiation of these cells into granulocytes is associated with receptor loss. Here we demonstrate that differentiation of HL-60 cells into monocytes or granulocytes induced by 1;25(OH)2vitD3 or Bt2cAMP, respectively, has no major effect on the specific activity of the insulin receptor kinase (IRK). By contrast, when HL-60 cells are incubated with a combination of 1;25(OH)2vitD3 and Bt2cAMP, their differentiation into adherent macrophages-like cells is accompanied by a 50% reduction in the specific activity of IRK. These findings suggest that acquisition or loss of insulin receptors during differentiation of HL-60 involves selective alterations in the functional aspects of these receptors. Our results also implicate the generation of specific regulatory signals that inhibit IRK activity when HL-60 cells are stimulated with a combination of 1;25(OH)2vitD3 and Bt2cAMP.

Bucladesine↗
PubMed1990

The insulinomimetic agents H2O2 and vanadate stimulate protein tyrosine phosphorylation in intact cells.

H2O2 and vanadate are known insulinomimetic agents. Together they induce insulin's bioeffects with a potency which exceeds that seen with insulin, vanadate, or H2O2 alone. Employing Western blotting with anti-P-Tyr antibodies, we have identified in Fao cells at least four proteins (pp180, 150, 114, and 100) whose P-Tyr content is rapidly increased upon treatment of the cells with 3 mM H2O2. Tyrosine phosphorylation of these and additional proteins was markedly potentiated (6-10-fold) when 100 microM sodium orthovanadate was added together with H2O2. The effects of H2O2 and vanadate on protein tyrosine phosphorylation were rapid and specific. The enhanced tyrosine phosphorylation was accompanied by a concomitant inhibition of a cytosolic protein tyrosine phosphatase activity. The latter was inhibited by 50% in 3 mM H2O2-treated cells. The inhibitory effect was augmented in the combined presence of H2O2 and vanadate. Half- and maximal effects of vanadate were obtained at 15 microM and 1 mM, respectively. Vanadate (1 mM) alone, added to the cells, had only a trivial effect on protein tyrosine phosphatase activity. A 45-s challenge with insulin (10(-7) M) of cells pretreated with H2O2 largely mimicked the potentiating effects of vanadate on protein tyrosine phosphorylation but not on protein tyrosine phosphatase activity. Our results suggest the involvement of multiple tyrosine-phosphorylation proteins in mediating the biological effects of H2O2/vanadate. Their enhanced phosphorylation can be attributed at least in part, to the inhibitory effects exerted by H2O2 alone, or in combination with vanadate, on protein tyrosine phosphatase activity. The similarity between proteins phosphorylated in Fao cells in response to H2O2/vanadate or H2O2/insulin, suggests that either treatment stimulates protein tyrosine kinases having similar substrate specificities. The insulin receptor kinase is a likely candidate as its activity is markedly enhanced either by insulin (plus H2O2) or by H2O2/vanadate.

Animals↗
PubMed1989

Antibodies directed against phosphothreonine residues as potent tools for studying protein phosphorylation.

Here we report the development of novel antibodies which specifically react with phosphothreonine residues [anti-(P-Thr)antibodies]. The specificity of the antibodies was assessed in radioimmunoassays where we could demonstrate that half-maximal and maximal binding of the antibodies to plates coated with BSA - P-Thr occurred at serum dilutions of 1:4000 and 1:1000, respectively. P-Thr inhibited antibody binding with a half-maximal effect at 40 microM. P-Ser was 200-fold less potent while P-Tyr was essentially ineffective. Anti-(P-Thr) antibodies could specifically bind to phosphothreonine-containing proteins on Western blots. Using such a procedure we could demonstrate enhanced threonine phosphorylation of the EGF receptor upon treatment of intact unlabeled A431 cells with EGF. We could further demonstrate antibodies binding to proteins present in extracts of rat hepatoma cells (Fao). P-Thr at 10 microM completely inhibited antibody binding while P-Ser, P-Tyr, Thr or Ser, each present at tenfold higher concentrations, had no such inhibitory effect. Anti-(P-Thr) antibodies were also capable of specifically immunoprecipitating 32P-labeled phosphoproteins present in Triton extracts of Fao cells. Immunoprecipitation of proteins of 38 kDa, 55 kDa, 85 kDa, 100 kDa and 155 kDa was inhibited by 1 mM P-Thr but not by P-Tyr. These findings suggest that anti-(P-Thr) antibodies could be powerful tools in studies aimed at monitoring alterations in threonine phosphorylation of specific proteins as they occur under physiological conditions in response to various extracellular stimuli. Identification of such proteins can be conveniently monitored by immunoblotting.

Animals↗
PubMed1989

H2O2 potentiates phosphorylation of novel putative substrates for the insulin receptor kinase in intact Fao cells.

Western blotting with anti-phosphotyrosine antibodies was employed in order to study insulin-dependent protein tyrosine phosphorylation in intact Fao cells. In insulin-treated cells, a prominent 180-kDa protein underwent tyrosine phosphorylation, which peaked at 45 s and then rapidly declined. Pretreatment of the cells with 1 mM Bt2cAMP or 0.16 microM 12-O-tetradecanoylphorbol-13-acetate inhibited the insulin-dependent phosphorylation of pp 180, while 1 mM vanadate or 3 mM H2O2 markedly potentiated it. These results indicate that phosphorylation of pp 180 is respectively regulated by agents that are known to synergize with or antagonize the action of the insulin receptor kinase. pp 180 is therefore likely to mediate physiological functions of this receptor kinase. Incubation of Fao cells with 3 mM H2O2 for 30 min prior to their treatment with insulin for 45 s allowed the detection of additional, previously undescribed, proteins pp 150, 114, 100, 85, 68, and 56 kDa that underwent insulin-dependent tyrosine phosphorylation. The potentiating effects of H2O2 were time- and dose-dependent and could be reversed by 2 mM dithiothreitol. Proteins phosphorylated in response to H2O2 plus insulin maintained their fully phosphorylated state for at least 20 min. We suggest that these phosphoproteins are potential physiological substrates for the insulin receptor kinase.

Bucladesine↗
PubMed1989

Studying the structure of the intracellular moiety of the insulin receptor with a kinase-splitting membranal proteinase.

A kinase-splitting membranal proteinase specifically clips the cytoplasmic moiety of the insulin receptor beta-subunit (95 kd) to yield an 84-kd fragment. Using antibodies against different domains in the receptor, cleavage is shown to remove an 11-kd 'tail' (rooted at the C-terminal end of the kinase domain) which includes tyrosines 1316 and 1322. This cleavage impairs the ability of the clustered tyrosines 1146, 1150 and 1151 to undergo autophosphorylation. Nevertheless, the clipped beta-subunit is as active as the intact subunit if its kinase activity is measured at high exogenous substrate concentrations (greater than or equal to 2 mg/ml) indicating that autophosphorylation is not obligatory for insulin-dependent phosphotransferase activity. With low substrate concentrations (e.g. 0.2 mg/ml) a severe damage to the kinase activity is detected, which may reflect an important structural contribution of the 'tail' and/or the clustered phosphotyrosines in creating the preferential affinity of the kinase for its in vivo substrate(s). The membranal proteinase strictly recognizes the native conformation of the kinase domain, and fails to cleave it after denaturation. Since such a conformation-dependent cleavage occurs also in the case of the cytoplasmic moiety of the EGF receptor and the catalytic subunit of cAMP-dependent protein kinase, it is suggested that the similarity between these three kinase domains extends beyond their reported sequence homology to reflect a similarity in conformation.

Animals↗
PubMed1989

Insulin and IGF-I stimulate phosphorylation of their respective receptors in intact neuronal and glial cells in primary culture.

Previous studies have shown that insulin and IGF-I bind to their respective receptors and stimulate autophosphorylation of the receptor beta subunits in detergent extracts of neuronal and glial cells. In the present study, intact neuronal and glial cells in primary culture have been utilized to characterize insulin- and IGF-I-stimulated phosphorylation of their receptors. Following [32P]orthophosphate labelling and stimulation by insulin or IGF-I, the cells were solubilized and the phosphorylated receptors were partially purified on wheat germ agglutinin--agarose columns, and immunoprecipitated using anti-phosphotyrosine or anti-insulin receptor antibodies. Insulin stimulated the phosphorylation of its receptor beta subunit (95 kD phosphoprotein) in a dose-dependent manner, within at least 20 seconds in both neuronal and glial cells. Additionally, a 102-kD phosphoprotein was observed in insulin-stimulated neuronal cells. Maximal stimulation of receptor phosphorylation occurred at 1 minute for the glial cells, and 10 minutes for the neuronal cells. IGF-I stimulated the phosphorylation of two phosphoproteins in intact neuronal and glial cells; a 95-kD protein and a 102-kD protein, in a dose-dependent manner. These observations demonstrate that both insulin and IGF-I stimulate the phosphorylation of the beta subunits of their respective receptors in brain cells in a similar fashion to their effects on receptors from nonneural tissues.

Animals↗
PubMed1989

Protein kinase C-mediated phosphorylation of retinal rod outer segment membrane proteins.

We have previously reported that the purified GDP-bound alpha-subunit of the GTP-binding protein transducin (TD), present in outer segments of retinal rod cells (ROS), serves as a high affinity substrate (Km = 1 microM) for protein kinase C (PKC) [Zick et al. (1986) Proc. natn. Acad. Sci., U.S.A. 83, 9294-9297]. In the present study we demonstrate that TD-alpha undergoes phosphorylation by PKC when present in its native form in intact ROS membranes. This phosphorylation is inhibited by GTP-gamma-S which activates TD, suggesting that it is only the inactive conformation of TD-alpha that serves as a substrate for PKC. Indeed, both vanadate and AlF4, that confer an active conformation on TD-alpha-GDP, inhibit PKC-mediated phosphorylation of purified TD-alpha-GDP. We demonstrate that the purified beta subunit of TD also serves as an in vitro substrate for PKC. Moreover, following their phosphorylation, both TD-alpha and beta form high affinity complexes with PKC. This is evident from the findings that PKC coprecipitates with both the alpha and beta subunits of TD when the latter are immunoprecipitated by their respective antibodies. PKC phosphorylates additional ROS proteins of 36, 48 and 92 kDa, tentatively identified as rhodopsin, arrestin and the cGMP-phosphodiesterase. Taken together our results strongly suggest that phosphorylation of TD is of physiological relevance and that through phosphorylation of endogenous ROS proteins, PKC could play a key role in regulating phototransduction.

Animals↗