Cytoskeletal Interactions of Raplb in Platelets

Low molecular weight GTP binding proteins (G proteins) are membrane-associated proteins which reversibly bind guanine nucleotides and regulate cellular processes, such as growth and differentiation (Evans et al., 1991; Macara, 1991). Members of this superfamily of proteins show considerable sequence homology and share structural features, including an effector domain which interacts with GTPase activating proteins or GAPs and post-translational modification at the carboxy terminus by polyisoprenyl groups, either farnesyl or geranyl-geranyl (Maltese, 1990; Gibbs, 1991). To date, more than 50 low molecular weight G proteins in four subfamilies have been reported. The prototype for this group of proteins is p21ras the 21 kDa protein product of the ras protooncogene. At least seven distinct G proteins are present in platelets (Bhullar & Haslam, 1988; Ohmori et al., 1988; Polakis et al., 1989; Polakis et al., 1989; Farrell et al., 1990; Nemoto et al., 1992) (Table I).