p50(cdc37) acting in concert with Hsp90 is required for Raf-1 function

Genetic screens in Drosophila have identified p50(cdc37) to be an essential component of the sevenless receptor/mitogen-activated kinase protein (MAPK) signaling pathway, but neither the function nor the target of p50(cdc37) in this pathway has been defined. In this study, we examined the role of p50(cdc37) and its Hsp90 chaperone partner in Raf/Mek/MAPK signaling biochemically. We found that coexpression of wild-type p50(cdc37) with Raf-1 resulted in robust and dose-dependent activation of Raf-1 in Sf9 cells. In addition, p50(cdc37) greatly potentiated v-Src-mediated Raf-1 activation. Moreover, we found that p50(cdc37) is the primary determinant of Hsp90 recruitment to Raf-1. Overexpression of a p50(cdc37) mutant which is unable to recruit Hsp90 into the Raf-1 complex inhibited Raf-1 and MAPK activation by growth factors. Similarly, pretreatment with geldanamycin (GA), an Hsp90-specific inhibitor, prevented both the association of Raf-1 with the p50(cdc37)-Hsp90 heterodimer and Raf-1 kinase activation by serum. Activation of Raf-1 via baculovirus coexpression with oncogenic Src or Ras in Sf9 cells was also strongly inhibited by dominant negative p50(cdc37) or by GA. Thus, formation of a ternary Raf-1-p50(cdc37)-Hsp90 complex is crucial for Raf-1 activity and MAPK pathway signaling. These results provide the first biochemical evidence for the requirement of the p50(cdc37)-Hsp90 complex in protein kinase regulation and for Raf-1 function in particular.