Regulation of adaptor protein GIT1 in platelets, leading to the interaction between GIT1 and integrin αIIbβ3

GIT1 is an adaptor protein, which links signaling proteins to focal adhesion, thereby regulating cytoskeletal reorganization. Platelets undergo dynamic cytoskeletal reorganization during platelet activation, for which a large number of adaptor proteins are required. However, there has been no report of GIT1 in platelets. We found that GIT1 was abundantly expressed in platelets and underwent tyrosine phosphorylation downstream of integrin alpha(IIb)beta(3), which was inhibited by the Src kinase inhibitor PP2. Furthermore, GIT1 constitutively associated with beta PIX, a guanine nucleotide exchange factor (GEF) for Rac. The GIT1/beta PIX complex associated with alpha(IIb)beta(3), concomitantly with GIT1 tyrosine phosphorylation. Moreover, both GIT1 and alpha(IIb)beta(3) rapidly translocated to the cytoskeletal fraction during platelet aggregation, which was not observed in the absence of aggregation. These results suggest that tyrosine phosphorylation of GIT1 by Src kinases may regulate cytoskeletal reorganization downstream of alpha(IIb)beta(3) by bringing the Rac GEF beta PIX to the vicinity of the integrin. (c) 2008 Elsevier Inc. All rights reserved.