The inadvertent activation of the Abelson tyrosine kinase (Abl) causes chronic myelogenous leukemia (CML). A small-molecule inhibitor of Abl (STI-571) is effective in the treatment of CML. We report the crystal structure of the catalytic domain of Abl, complexed to a variant of STI-571. Critical to the binding of STI-571 is the adoption by the kinase of an inactive conformation, in which a centrally located "activation loop" is not phosphorylated. The conformation of this loop is distinct from that in active protein kinases, as well as in the inactive form of the closely related Src kinases. These results suggest that compounds that exploit the distinctive inactivation mechanisms of individual protein kinases can achieve both high affinity and high specificity.
The crystal structure of the autoinhibited form of Hck has been determined at 2.0 A resolution, in complex with a specific pyrazolo pyrimidine-type inhibitor, PP1. The activation segment, a key regulatory component of the catalytic domain, is unphosphorylated and is visualized in its entirety. Tyr-416, the site of activating autophosphorylation in the Src family kinases, is positioned such that access to the catalytic machinery is blocked. PP1 is bound at the ATP-binding site of the kinase, and a methylphenyl group on PP1 is inserted into an adjacent hydrophobic pocket. The enlargement of this pocket in autoinhibited Src kinases suggests a route toward the development of inhibitors that are specific for the inactive forms of these proteins.
Here we used the cold-shock protein CspB from Bacillus subtilis to study protein folding at an elementary level. The thermodynamic stability of this small five-stranded beta-barrel protein is low, but unfolding and refolding are extremely rapid reactions. In 0.6 M urea the time constant of refolding is about 1.5 ms, and at the transition midpoint (4 M urea) the folded and unfolded forms equilibrate in less than 100 ms. Both the equilibrium unfolding transition and the folding kinetics are perfectly described by a N<-->U two-state model. The validity of this model was confirmed by several kinetic tests. Folding intermediates could neither be detected at equilibrium nor in the folding kinetics. We suggest that the extremely rapid folding of CspB and the absence of folding intermediates are related phenomena.
Summary
In contrast to the active conformations of protein kinases, which are essentially the same for all kinases, inactive kinase conformations are structurally diverse. Some inactive conformations are, however, observed repeatedly in different kinases, perhaps reflecting an important role in catalysis. In this review, we analyze one of these recurring conformations, first identified in CDK and Src kinases, which turned out to be central to understanding of how kinase domain of the EGF receptor is activated. This mechanism, which involves the stabilization of the active conformation of an α helix, has features in common with mechanisms operative in several other kinases.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.