Thy-1 associated pp85--90 is a potential docking site for SH2 domain-containing signal transduction molecules

Thy-1, a glycosylphosphatidylinositol (GPI)-anchored glycoprotein expressed at high levels on thymocytes, has been implicated in positive and negative signal transduction. We show that Thy-1 associates with a protein of 85--90 kDa, which is prominently phosphorylated in vitro as well as in vivo following the stimulation of thymocytes with pervanadate. pp85--90 is not identical to known proteins that are phosphorylated following T cell activation. The SH2 domains of fyn, csk, phosphatidylinositol 3'-kinase, rasGAP, vav and lck bind to pp85--90 with varying affinities. The SH2 domains of ZAP70, SHP-1 and PLC gamma 1 and the SH3 domains of lck, vav and HS1 did not bind to pp85--90. The molecular weight, iso-electric point, efficient phosphorylation by fyn and lck and preferential binding to the SH2 domain of fyn compared to that of lck indicate that Thy-1-associated pp85-90 may be identical to a recently cloned, fyn-associated transmembrane adaptor protein, PAG-85.