The related adhesion focal tyrosine kinase differentially phosphorylates p130(Cas) and the Cas-like protein, p105(HEF1)

The related adhesion focal tyrosine kinase (RAFTK) is tyrosine-phosphorylated following beta 1 integrin or B cell antigen receptor stimulation in human B cells. Two substrates that are tyrosine-phosphorylated following integrin ligation in B cells are p130(Cas) and the Cas family member human enhancer of filamentation 1 (HEF1), both of which can associate with RAFTK. In this report we observed that RAFTK was involved in the phosphorylation of these two proteins. While a catalytically active RAFTK was required for both p130(Cas) and HEF1, phosphorylation of p130(Cas), but not of HEF1, was dependent on an intact autophosphorylation site (Tyr(402)) on RAFTK. To determine if RAFTK phosphorylated p130(Cas) and HEF1 directly or through an intermediate, we assayed the ability of RAFTK and of a Tyr(402) mutant to phosphorylate purified HEF1 and p130(Cas) domains. RAFTK was able to phosphorylate the substrate domains of both p130(Cas) and HEF1, but only the C-terminal domain of p130(Cas). Furthermore, Tyr(402), which mediates the binding of RAFTK to c-Src kinase, was required for the phosphorylation of the C-terminal domain of p130(Cas). These data suggest that RAFTK itself is sufficient for HEF1 phosphorylation, whereas a cooperation between RAFTK and Src kinases is required for the complete phosphorylation of p130(Cas).