Ab1 N-terminal Cap stabilization of SH3 domain dynamics

Crystal structures and other biochemical data indicate that the N-terminal cap (NCap) region of the Abelson tyrosine kinase (c-Ab1) is important for maintaining the downregulated conformation of the kinase domain. The exact contributions that the NCap makes in stabilizing the various intramolecular interactions within c-Ab1 are less clear. While the NCap appears to be important for locking the SH3 and SH2 domains to the back of the kinase domain, there may be other more subtle elements of regulation. Hydrogen exchange (HX) and mass spectrometry (MS) were used to determine if the NCap contributes to intramolecular interactions involving the Ab1 SH3 domain. Under physiological conditions, the Ab1 SH3 domain underwent partial unfolding and its unfolding half-life was slowed during binding to the SH2 kinase linker, providing a unique assay for testing NCap-induced stabilization of the SH3 domain in various constructs. The results showed that the NCap stabilizes the dynamics of the SH3 domain in certain constructs but does not increase the relative affinity of the SH3 domain for the native SH2 kinase linker. The stabilization effect was absent in constructs of just the NCap and SH3 but was obvious when the SH2 domain and the SH2 kinase linker were present. These results suggest that interactions between the NCap and the SH3 domain can contribute to c-Ab1 stabilization in constructs that contain at least the SH2 domain, an effect that may partially compensate for the absence of the negative regulatory C-terminal tail found in the related Src family of kinases.