Inhibition of the Activity of Src and Abl Tyrosine Protein Kinases by the Binding of the Wiskott−Aldrich Syndrome Protein

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Abstract
The Wiscott−Aldrich syndrome protein, WASP, is an effector through which cdc42, a Rho family GTPase, regulates the actin cytoskeleton in hematopoietic cells. We have found that WASP binds readily to a number of tyrosine protein kinases including the Src kinases and the Abl kinase when the proteins are coexpressed during transient transfection. Binding inhibited the activity of each of these kinases strikingly, both in vitro and in vivo. Surprisingly, the binding was not due to an interaction between the proline-rich domain of WASP and the SH3 domain of these kinases. Rather, residues 83−93 in WASP were found to bind to the catalytic domains of the kinases. Binding did not decrease the affinity of Src kinases for either ATP or a peptide substrate noticeably. Rather, the Vmax of substrate phosphorylation was reduced by the binding of the peptide. This inhibition represents a novel form of regulation of protein kinase activity and suggests that that the isolation of small molecules that exploit this inhibitory mechanism may be possible.

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