| The c-Abl non-receptor tyrosine kinase has been implicated in a variety of cellular processes, yet its function and regulation remain poorly understood. Among the large number of proteins and factors shown to interact with and possibly regulate c-Abl is the SH2/SH3 adaptor protein, Nck.; We have found that a construct encoding the first two SH3 domains of Nck can activate c-Abl in fibroblasts. Activation by both myristoylated and unmyristoylated Nck SH3 domains required the C-terminus of Abl. More specific analysis of the C-terminus showed that either the major binding site for Nck SH3 domains or its actin-binding domain is sufficient to mediate AN activation. Addition of either the Nck binding site or the Abl actin-binding domain to C-terminally truncated Abl restored activation. We propose that Abl activation is occurring because the SH3 domains of Nck compete with Abl's own inhibitory SH3 domain for binding to its SH2-linker region. This activation by Nck SH3 domains is facilitated by high local concentrations of Abl via membrane localization and/or association with the actin cytoskeleton. In addition, we have uncovered a role for Abl in affecting localized areas of the cytoskeleton by stimulating the formation of filopodial-like structures.; Cbl has been previously shown to be a substrate of the non-receptor tyrosine kinase c-Abl, but the functional significance of this interaction has yet to be determined. We found that normally inactive wild-type c-Abl phosphorylates Cbl when co-expressed in fibroblasts. Subsequent experiments suggest that cell-cell contact or cell shape may play an important role in regulating the phosphorylation of Cbl by c-Abl.; These studies shed new light on the biological role of Abl, as well as the molecular mechanism behinds its activation and regulation. As Abl is proposed to regulate and be regulated by the actin cytoskeleton, further characterization of the interaction between Abl/Nck and Abl/Cbl may provide insight into the understanding of Abl activity in a wider biological context. |
