Interactions between two cytoskeleton-associated tyrosine kinases: Calcium-dependent tyrosine kinase and focal adhesion tyrosine kinase

The calcium-dependent tyrosine kinase (CADTK), also known as Pyk2/RAFTK/CAK beta/FAX2, is a cytoskeleton-associated tyrosine kinase. We compared CADTK regulation with that of the highly homologous focal adhesion tyrosine kina se (FAK). First, we generated site-specific CADTK mutants. Mutation of Tyr( 402) eliminated autophosphorylation and significantly decreased kinase acti vity. Mutation of Tyr(881), a putative Src kinase phosphorylation site pred icted to bind Grb2, had little effect on CADTK regulation. Src family tyros ine kinases resulted in CADTK tyrosine phosphorylation even when co-express ed with the Tyr(402)/Tyr(881) double mutant, suggesting that Src/Fyn etc. p hosphorylate additional tyrosine residues. Interestingly, CADTK tyrosine-ph osphorylated FAK when both were transiently expressed, but FAK did not phos phorylate CADTK. Biochemical experiments confirmed direct CADTK phosphoryla tion of FAK. This phosphorylation utilized tyrosine residues other than Tyr (397), Tyr(925), Or Tyr(576)/Tyr(577), suggesting that new SH2-binding site s might be created by CADTK-dependent FAK phosphorylation. Last, expression of the CADTK carboxyl terminus (CRNK) abolished CADTK but not FAK autophos phorylation. In contrast, FAK carboxyl terminus overexpression inhibited bo th FAK and CADTK autophosphorylation, suggesting that a FAK-dependent cytos keletal function may be necessary for CADTK activation. Thus, CADTK and FAK , which both bind to some, but not necessarily the same, cytoskeletal eleme nts, may be involved in coordinate regulation of cytoskeletal structure and signaling.