Signaling in Anergy: from Fyn to Ras and beyond DFG FOR 521

The phosphoprotein associated with glycosphingolipid-enriched microdamins (PAG) is a transmembrane adaptor protein that negatively regulates the activity of Src family kinases by recruiting the cytosolic C-terminal Src kinase (Csk) to the plasma membrane where Csk phosphorylates the negative regulatory tyrosine conserved within the C-terminus of all Src family kinases. In this report, we extend the knowledge of PAG by demonstrating a role for lipid raft targeting on PAG function. In addition to its ability to negatively regulate Src kinases, we have also shown that PAG negatively regulates Ras. This is accomplished through the formation of a multi-protein complex containing PAG, Fyn, Sam68, and p120RasGAP. We believe that in this way PAG contributes to the block in Ras activation found in anergic T-cells. While investigating proximal signaling events in anergic T-cells, we also serendipitously observed the strong induction of a 120 kDa protein on Western blots. Since no known isoforms of the antigen exist, we isolated this protein and have identified it as a novel Ras GTPase-activating protein (GAP). Our aim is to characterize this new inducible-GAP (named IGAP).