EFFECT OF CAMP ON THE ASSOCIATION OF SMALL GTP-BINDING PROTEINS WITH THE CYTOSKELETON OF HUMAN PLATELETS

Following activation of human platelets changes in cytoskeletal organi zation occur: some proteins, which are present in the cytosol or membr ane-associated in resting platelets, are recovered in the Triton-insol uble residue in activated cells. Assembly and disassembly of complex e ffector units on the membrane and inside cells is under the control of low molecular weight GTP-binding proteins, particularly those in the ras family. We investigated the interaction of small GTP-binding prote ins with the platelet cytoskeleton and the effect of high cAMP levels on these interactions. At least two GTP-binding proteins of 24 and 28 kDa were detected in the Triton-insoluble residue of resting platelets . Stimulation of platelets with thrombin or concanavalin A (Con A), un der non-aggregating conditions, resulted in increased 24 kDa protein-b ound GTP, which also contained a significant amount of rap1B. High cAM P levels differently affected this interaction depending on the type o f agonist used, cAMP increased association of G-proteins with the cyto skeleton following Con A-activation, while it decreased G-proteins int eraction after thrombin stimulation. The activation did not influence the cAMP-dependent phosphorylation of rap1B. No phosphoprotein corresp onding to rap1B could be detected in the Triton-insoluble residues, ho wever. These findings could be related to the different mechanisms of cytoskeletal protein recruitment in platelets activated with either th rombin or Con A.