The regulatory mechanism of a client kinase controlling its own release from Hsp90 chaperone machinery through phosphorylation

It is believed that the stability and activity of client proteins are passively regulated by the Hsp90 (heat-shock protein 90) chaperone machinery, which is known to be modulated by its intrinsic ATPase activity, co-chaperones and post-translational modifications. However, it is unclear whether client proteins themselves participate in regulation of the chaperoning process. The present study is the first example to show that a client kinase directly regulates Hsp90 activity, which is a novel level of regulation for the Hsp90 chaperone machinery. First, we prove that PKC gamma (protein kinase C gamma) is a client protein of Hsp90 alpha, and, that by interacting with PKC gamma, Hsp90 alpha prevents PKC gamma degradation and facilitates its cytosol-to-membrane translocation and activation. A threonine residue set, Thr(115)/Thr(425)/Thr(603), of Hsp90 alpha is specifically phosphorylated by PKC gamma, and, more interestingly, this threonine residue set serves as a 'phosphorylation switch' for Hsp90 alpha binding or release of PKC gamma, Moreover, phosphorylation of Hsp90 alpha by PKC gamma decreases the binding affinity of Hsp90 alpha towards ATP and co-chaperones such as Cdc37 (cell-division cycle 37), thereby decreasing its chaperone activity. Further investigation demonstrated that the reciprocal regulation of Hsp90 alpha and PKC gamma plays a critical role in cancer cells, and that simultaneous inhibition of PKC gamma and Hsp90 alpha synergistically prevents cell migration and promotes apoptosis in cancer cells.