The antigenicity of platelet proteins was assayed against various monoclonal antibodies (mAbs) that recognize specific epitopes of the ras-encoded p21 protein. mAb M90, which detects the region of p21 protein within amino acids 107-130 and inhibits its GTP-binding activity, strongly reacted with a 22-kDa protein present in the particulate fraction of human platelets. Other mAbs against ras-encoded proteins, including Y13-259, which efficiently detects ras proteins from a variety of organisms, did not recognize the platelet 22-kDa protein. Transfer ofthe platelet 22-kDa protein to nitrocellulose paper showed that the protein binds [a-32P]GTP. Moreover, preincubation of the transferred protein with mAb M90 drastically reduced its GTP-binding activity. Treatment of platelets with ioprost, a prostacyclin analog, caused (i) a time-dependent increase of a 24-kDa protein that is recognized by mAb M90 in particulate and cytosolic fractions and (ii) the gradual decrease of the 22-kDa protein from the particulate fraction. When platelets were labeled with 32P and then treated with iloprost, the 24-kDa protein was found to be phosphorylated. The 32P-labeled 24-kDa protein was specifically immunoprecipitated by mAb M90. These results suggest that appearance of the 24-kDa protein results from phosphorylation of the 22-kDa protein, which shifts its mobility to a higher molecular mass area.In the past two years platelets have been shown to possess distinct GTP-binding proteins with molecular masses between 21 and 31 kDa (1-3). These low molecular weight guanine nucleotide-binding regulatory (G) proteins can be electrophoretically transferred from SDS/polyacrylamide gels to nitrocellulose blots, where they are detected by the binding of [a-32P]GTP (2, 3). One of them, a 21-kDa protein, was recently isolated from platelet membranes; however, this protein is not recognized with monoclonal antibody (mAb) Y13-259 (4), which recognizes all known ras-encoded p21 proteins (1). Also, this antibody did not cross-react with the other low molecular mass G proteins present in platelets (3). The possible physiological significance of these proteins has not yet been elucidated; they could be correlated with regulation of phospholipase C (2) and, recently, we reported that a platelet ras-related protein is phosphorylated by a cAMP-dependent protein kinase (5).Lacal and Aaronson (6) have developed a series of mAbs that recognize epitopes located at different regions in the Harvey (Ha) ras-encoded p21 protein. We have screened platelet proteins against those mAbs and found that a 22-kDa platelet protein is recognized by two of those mAbs. These mAbs, designated M3 and M90, recognize a region of ras p21 protein containing residues 116-119, involved in the interaction with the guanine base (7). By contrast the 22-kDa protein was not recognized by the ras-specific mAb Y13-259. These results suggest that the 22-kDa platelet protein is a member ofthe ras family. Finally, the 22-kDa protein is a substrate for cAMP-dependent protein kinase. Ph...

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Treatment of platelets with a prostacyclin analogue, iloprost, decreased the cholera-toxin-induced ADP-ribosylation of membrane-bound Gs alpha (alpha-subunit of G-protein that stimulates adenylate cyclase; 42 kDa protein) and a cytosolic substrate (44 kDa protein) [Molina y Vedia, Reep & Lapetina (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 5899-5902]. This decrease is apparently not correlated with a significant change in the quantity of membrane Gs alpha, as detected by two Gs alpha-specific antisera. This finding contrasts with the suggestion in a previous report [Edwards, MacDermot & Wilkins (1987) Br. J. Pharmacol. 90, 501-510], indicating that iloprost caused a loss of Gs alpha from the membrane. Our evidence points to a modification in the ability of the 42 kDa protein to be ADP-ribosylated by cholera toxin. This modification of Gs alpha might be related to its ADP-ribosylation by endogenous ADP-ribosyltransferase activity. Here we present evidence showing that Gs alpha was ADP-ribosylated in platelets that had been electropermeabilized and incubated with [alpha-32P]NAD+. This endogenous ADP-ribosylation of Gs alpha is inhibited by nicotinamide and stimulated by iloprost.

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