pp60(c-src) associates with the SH2-containing inositol-5-phosphatase SHIP1 and is involved in its tyrosine phosphorylation downstream of alpha IIb beta 3 integrin in human platelets

SH2-containing inositol-5-phosphatase 1 (SHIP1) was originally identified a s a 145 kDa protein that became tyrosine-phosphorylated in response to mult iple cytokines. It is now well established that SHIP1 is specifically expre ssed in haemopoietic cells and is important as a negative regulator of sign alling. We found recently that SHIP1 was present in human blood platelets a s an Ins(1,3,4,5)P-4-phosphatase and a PtdIns(3,4,5)P-3-5-phosphatase that became tyrosine-phosphorylated and was relocated to the cytoskeleton in an integrin-dependent manner. Here we report biochemical and pharmacological e vidence that the tyrosine kinase pp60(c-arc) is constitutively associated w ith SHIP1 and, is involved in its tyrosine phosphorylation downstream of in tegrin engagement in thrombin-activated human platelets. The use of cytocha lasin D allowed us to demonstrate that the actin cytoskeleton reorganizatio n induced on thrombin stimulation was not required for its integrin-mediate d phosphorylation. Moreover, the integrin-dependent relocation of SHIP1 to the cytoskeleton did not require its tyrosine phosphorylation. These result s suggest that SHIP1 is first recruited to the integrin-linked signalling c omplexes and then becomes tyrosine-phosphorylated through a Src-kinase-depe ndent mechanism but independently of the actin cytoskeleton reorganization.