Tight control of the activity of the 518 human kinases is crucial for regulating cellular processes from cell division to energy metabolism. Thus, it is not surprising that kinases are directly or indirectly involved in many diseases and that kinases are key drug targets.
For example, Src kinase was the first identified proto-oncogene, and the formation of a de-regulated Abl fusion protein (BCR-Abl) is the underlying cause of disease in 95% of patients with chronic myeloid leukemia. X-ray crystal structures have shown that the same kinase domain can adopt an actcive and various inactive conformations (Figure 1). This implies that kinases are inherently flexible and that inactive and active kinase conformations exist in a dynamic equilibrium. How the active and inactive states are stabilized and how states interconvert are key questions in understanding kinase regulation. BCR-Abl is the target of the clinically highly successful drug imatinib (Gleevec, Novartis) in the treatment of chronic myelogenous leukemia (CML). The excellent specificity of imatinib, which significantly inhibits only Abl and three other kinases out of 518, contributes considerably to its success. This specificity is due to the fact that imatinib binds only one inactive kinase conformation of Abl. Therefore drug binding must be intimately related to the interconversion between active and inactive kinase states; in other words, processes that prevent the kinase from accessing a particular inactive conformation will also prevent imatinib binding. Since clinical resistance mutations to imatinib can perturb this key interconversion process, the proposed dynamic studies will yield data directly relevant the mechanism of drug resistance.
To complement insights from existing X-ray crystal structures, we will use NMR experiments and ligand binding kinetics to study the timescales and amplitudes of structural interconversions in Abl and Src kinase domains.
We are also interested in the mechanism of novel kinase inhibitors and we are collaborating closely with a number of medicinal chemists on this project.
We gratefully acknowledge support for this project by the National Institutes of Health, National Institute for General Medical Sciences.