The p85 subunit of phosphatidylinositol 3-kinase associates with the Fc receptor gamma-chain and linker for activitor of T cells (LAT) in platelets stimulated by collagen and convulxin.

There is extensive evidence to show that phosphatidylinositol 3-kinase plays an important role in signaling by the immune family of receptors, which has recently been extended to include the platelet collagen receptor, glycoprotein VI. In this report we present two potential mechanisms for the regulation of this enzyme on stimulation of platelets by collagen. We show that on stimulation with collagen, the regulatory subunit of phosphatidylinositol 3-kinase associates with the tyrosine-phosphorylated form of the adapter protein linker for activator of T Cells (LAT) and the tyrosine-phosphorylated immunoreceptor tyrosine-based activation motif of the Fc receptor gamma-chain (a component of the collagen receptor complex that includes glycoprotein VI). The associations of the Fc receptor gamma-chain and LAT with p85 are rapid and supported by the Src-homology 2 domains of the regulatory subunit. We did not obtain evidence to support previous observations that the regulatory subunit of phosphatidylinositol 3-kinase is regulated through association with the tyrosine kinase Syk. The present results provide a molecular basis for the regulation of the p85/110 form of phosphatidylinositol 3-kinase by GPVI, the collagen receptor that underlies activation.

Differential roles for the adapters Gads and LAT in platelet activation by GPVI and CLEC-2.

The results demonstrate that Gads plays a key role in linking the adapter LAT to SLP-76 in response to weak activation of GPVI and CLEC-2 whereas LAT is required for full activation over a wider range of agonist concentrations. These results reveal the presence of a Gads-independent pathway of platelet activation downstream of LAT.