Low molecular weight GTP-binding proteins are involved in the regulation of several cellular functions, including transmembrane signaling, cell growth and differentiation, cytoskeletal organization, and secretion. These monomeric proteins range in mass from 20 to 30 kDa; they can bind GDP and GTP, and possess low intrinsic GTPase activity. In the past few years more than 50 distinct low molecular weight GTP-binding proteins have been identified, purified, and cloned from several mammalian tissues. Most of them are members of a large superfamily of proteins structurally related to the product of the protooncogene ras (1-3).

Two 1987 reports described several low molecular weight GTP-binding proteins in human platelets, that were able to bind [12P] GTP upon electrophoresis and transfer to nitrocellulose (4, 5). In subsequent years some of these proteins were recognized as known members of the ras superfamily of proteins. Today we know that at least nine distinct ras-related proteins are expressed in human platelets. Among these, there are members of the rap family of protems; these are the focus of this review.