A role for molecular chaperones in cell cycle entry

Cdk4 and Cdk2, together with there cyclin partners cyclin D and cyclin E, regulate passage through the G1/S transition. These enzymes are inactive in their monomeric form and require cyclin association and phosphorylation on a conserved threonine residue (T160 in Cdk2) by Cdk activating kinase (CAK) for full activity. Although the two-step mechanism of Cdk2 activation by cyclin A is relatively well understood in vitro , there is accumulating evidence that additional steps are involved in the production of active Cdk4 complexes in vivo and these steps may require the action of factors other than CAK. To better understand Cdk assembly in mammalian cells, we have characterized genes encoding the mammalian homologs of the budding yeast gene CDC37, previously implicated in Cdk assembly through an unknown mechanism. We find that mammalian p50cdc37 encodes a subunit of Hsp90 previously observed in association with Raf-1 and pp60v-src and that Cdk4 is a major target of Cdc37 in fibroblasts. Hsp90 is a molecular chaperone that functions primarily in signalling pathways involving steroid-hormone receptors and protein kinases in vivo. Cdc37 also has the properties of a protein kinase targeting subunit of Hsp90. Pharmacological disruption of Hsp90 function leads to the absence of Cdc37/Cdk4 complexes and a dramatic decrease in the stability of newly synthesized Cdk4, indicating that Cdk4 is intrinsically unstable and requires the Cdc37/Hsp90 chaperone for stabilization. Our results point to a previously unrecognized role for molecular chaperones in cell cycle control through stabilization of a Cdk required for the G1/S transition and identify a potential point of regulation in the assembly of Cdk4/cyclin D complexes.