Two distinct regions of the CD28 intracytoplasmic domain are involved in the tyrosine phosphorylation of Vav and GTPase activating protein-associated p62 protein

The T cell-associated CD28 molecule plays a key role in T cell co-stimulation. Its ligation induces the tyrosine phosphorylation of numerous proteins including CD28 itself as well as a restricted set of substrates of 97 and 62-68 kDa which are poorly phosphorylated by the tyrosine kinases induced by CD3-TCR triggering. In this study, we identify these substrates as the product of the vav proto-oncogene and as a 62 kDa protein that could correspond at least in part to p62^dok, the 62 kDa adaptor molecule associated to p120 Ras-GTPase activating protein. Both p97^vav and p62 are tyrosine phosphorylated upon CD28 ligation by mAb or by its counter-receptor B7-1/CD80. Using CD28 mutants, we also show that Vav and p62 tyrosine phosphorylation is regulated by distinct domains within the CD28 cytoplasmic tail: residues 173-181 for Vav and residues 182-202 for p62. Finally, the phosphorylation of Vav and p62 does not require an intact binding site for Grb-2 or p85 SH2 domains. We thus demonstrate that the CD28 cytoplasmic domain contains at least three functionally independent regions involved in CD28-induced signal transduction, since in addition to the Grb-2 and p85 SH2 domain binding site (Tyr173), residues 173-181 and 182-202 are associated with Vav and p62 tyrosine phosphorylation respectively.