Activation of the Abl tyrosine kinase in vivo by Src homology 3 domains from the Src homology 2/Src homology 3 adaptor Nck

The nonreceptor tyrosine kinase c-Abl is tightly regulated in vivo, but the mechanisms that normally repress its activity are not well understood, We find that a construct encoding the first two Src homology 3 (SH3) domains o f the Src homology 2/SH3 adaptor protein Nck can activate c-Abl in human 29 3T cells. A myristoylated Nck SH3 domain construct, which is expected to lo calize to membranes, potently activated Abl when expressed at low levels. A n unmyristoylated Nck SH3 domain construct, which localizes to the cytosol and nucleus, also activated Abl but only at high levels of expression. Acti vation by both myristoylated and unmyristoylated Nck constructs required th e C terminus of Abl; a C-terminally truncated form of Abl was not activated , although this construct could still be activated by deletion of its SH3 d omain. Activation did not require the major binding sites in the Abl C term inus for Nck SH3 domains, however, suggesting that the mechanism of activat ion does not require direct binding to the C terminus. Activation of c-Abl by Nck SH3 domains provides a robust experimental system for analyzing the mechanisms that normally repress Abl activity and how that normal regulatio n can be perturbed.