The nonreceptor tyrosine kinase ACK2, a specific target for Cdc42 and a negative regulator of cell growth and focal adhesion complexes.

ACK2 (activated Cdc42-associated tyrosine kinase-2) is a nonreceptor tyrosine kinase that is a specific target/effector for the GTP-binding protein Cdc42. Thus far the biological function of this tyrosine kinase has not been determined. Using an inducible eukaryotic expression system in fibroblasts, we demonstrate that ACK2 can strongly influence cell shape and growth as well as focal complex formation. ACK2 was found to associate with the focal adhesion complex components talin and vinculin, but not with the focal adhesion kinase (FAK), in a kinase-independent manner. The tyrosine kinase activity of FAK was also inhibited in cells overexpressing both wild-type and kinase-defective ACK2. This may be due to a competition between ACK2 and FAK for Src, which is an essential cofactor for FAK activation, as we have found that ACK2 specifically binds Src in cells. The ACK2-Src interaction appears to be mediated by the SH3 domain of Src, and the phosphorylation of ACK2 is enhanced in cells overexpressing the hyperactivated Src(Y527F) mutant. Overexpression of both wild-type and kinase-defective ACK2 also results in a severe inhibition of cell growth. In addition, ACK2 dissolves actin stress fibers and disassembles focal complexes but in a kinase-dependent manner. These results, taken together with previous studies demonstrating an association of ACK2 with integrin beta(1) (Yang, W., Lin, Q., Guan, J.-L., Cerione, R. A. (1999) J. Biol. Chem. 274, 8524-8530) and clathrin (Yang, W., Lo, C. G., Dispenza, T., and Cerione, R. A. (2001) J. Biol. Chem. 276, 17468-17473), suggest that the binding and protein tyrosine kinase activities of ACK2 coordinate changes in cell morphology and growth with the disassembly of focal adhesion sites, perhaps to organize new integrin complexes that are required for endocytosis and/or for cellular differentiation.