Cytoskeletal interactions of Rap1b in platelets.

We have presented evidence that rap1b, a 22 kDa low molecular weight GTP binding protein, becomes associated with the cytoskeleton in thrombin-activated platelets. The initial incorporation is very rapid and occurs as fast as we can measure it. Thus, some rap1b is associated with the cytoskeleton as fast as it is formed. The remainder of the rap1b is incorporated more slowly. This biphasic incorporation of rap1b is similar to the incorporation of GPIIb/IIIa into the cytoskeleton, but no interaction between GPIIb/IIIa and rap1b could be demonstrated. Phosphorylation of rap1b by cAMP-dependent protein kinase did not inhibit its association with the cytoskeleton. We conclude that rap1b is one of an increasing number of proteins that associate with the cytoskeleton during cell activation. The function of rap1b in the cytoskeleton is unclear at this time. However, it is possible to speculate on potential roles. There is growing evidence that low molecular weight G proteins participate in the formation of multi-molecular aggregates. For example, p21rac promotes the assembly of a membrane-associated complex composed of NADPH oxidase, p47, and p67 and this complex is important for activation of NADPH oxidase in neutrophils. Similarly, in yeast, BUD1, a homolog of rap1, forms a complex with BUD5 (a homolog of GDI), BEMI, CDC24, and CDC42 (a homolog of G25K). This multi-protein aggregate may be important in cytoskeletal structure in yeast. In platelets, rad1b, which is membrane associated, may promote the assembly of a complex of proteins during cell activation and may localize this complex to the plasma membrane.(ABSTRACT TRUNCATED AT 250 WORDS)