INTEGRIN-DEPENDENT TRANSLOCATION OF PHOSPHOINOSITIDE 3-KINASE TO THE CYTOSKELETON OF THROMBIN-ACTIVATED PLATELETS INVOLVES SPECIFIC INTERACTIONS OF P85-ALPHA WITH ACTIN-FILAMENTS AND FOCAL ADHESION KINASE

Thrombin-induced accumulation of phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P-2) but not of PtdIns(3,4,5,)P-3 is strongly correlated with the relocation to the cytoskeleton of 29% of the p85 alpha, regulatory subunit of phosphoinositide 3-kinase (PtdIns 3-kinase) and is accompanied by a significant increase in PtdIns 3-kinase activity in this subcellular fraction. Actually, PtdIns(3,4)P-2 accumulation and PtdIns 3-kinase, pp60(c-src), and p125(FAK) translocations as well as aggregation were concomitant events occurring with a distinct lag after actin polymerization. The accumulation of PtdIns(3,4)P-2 and the relocalization of PtdIns 3-kinase to the cytoskeleton were both dependent on tyrosine phosphorylation, integrin signaling, and aggregation. Furthermore, although p85 alpha was detected in anti-phosphotyrosine immunoprecipitates obtained from the cytoskeleton of thrombin-activated platelets, we failed to demonstrate tyrosine phosphorylation of cytoskeletal p85 alpha. Tyrphostin treatment clearly reduced its presence in this subcellular fraction, suggesting a physical interaction of p85 alpha with a phosphotyrosyl protein. These data led us to investigate the proteins that are able to interact with PtdIns 3-kinase in the cytoskeleton. We found an association of this enzyme with actin filaments: this interaction was spontaneously restored after one cycle of actin depolymerization-repolymerization in vitro. This association with F-actin appeared to be at least partly indirect, since we demonstrated a thrombin-dependent interaction of p85 alpha with a proline-rich sequence of the tyrosine-phosphorylated cytoskeletal focal adhesion kinase, p125(FAK). I, addition, we show that PtdIns 3-kinase is significantly activated by the p125(FAK) proline-rich sequence binding to the src homology 3 domain of p85 alpha subunit. This interaction may represent a new mechanism for PtdIns 3-kinase activation at very specific areas of the cell and indicates that the focal contact-like areas linked to the actin filaments play a critical role in signaling events that occur upon ligand engagement of alpha(IIb)/beta(3) integrin and platelet aggregation evoked by thrombin.