Product of vav proto-oncogene defines a new class of tyrosine protein kinase substrates

Abstract

SEVERAL proteins implicated in the regulation of cellular responses to mitogenic stimuli contain a common non-catalytic domain, SH2 (for src-homologous domain 2), that mediates their interaction with activated tyrosine protein kinases. Here we report that p95^vav, a proto-oncogene product specifically expressed in cells of the haematopoietic system, contains an SH2 domain and is a substrate for tyrosine protein kinases. Exposure of quiescent NIH3T3 cells ectopically expressing p95^vav to either epidermal or platelet-derived growth factors induces the rapid phosphorylation of this protein on tyrosine residues. Activation of the receptors for these growth factors by their cognate ligand results in their association with p95^vav, a process mediated by its SH2 domain. In T cells, co-activation of the T-cell receptor and the accessory CD4 cell-surface protein also results in the phosphorylation of the endogenous p95^vav protein in tyrosine residues. Phosphorylation of p95^vav is rapid, transient and precedes the appearance of most other phosphotyrosine-containing proteins. In addition to the SH2 domain, p95^vav contains structural motifs not found in other tyrosine kinase substrates. One such motif is a helix–loop–helix/leucine zipper-like domain which shares some sequence similarity with these motifs in the Myc and Max proteins. Deletion of the helix–loop–helix-like motif causes oncogenic activation of p95^vav. These results indicate that p95^vav is a new type of signal transduction molecule and suggest a possible role for this protein in the transduction of tyrosine phosphorylation signalling into transcriptional events.