Hsp70/Hsp90 Organizing Protein (HOP) Maintains CRAF Kinase Activity and Regulates MAPK Signaling by Enhancing Hsp90-CRAF Association.

The stability and activity of CRAF/Raf1 kinase are stringently regulated by heat shock protein 90 (Hsp90). Hsp90-mediated client folding and maturation are governed by its co-chaperones, but their functionality in chaperoning CRAF kinase to support signaling under physiological conditions remains poorly understood. Here, we show that Hsp70/Hsp90 organizing protein (HOP) associates with CRAF kinase tomaintain its activity and facilitates MAPK pathway activation. This activation is mediated by TPR2A-2B-DP2 domain of HOP and requires efficient binding to Hsp90. Although Cdc37 recruits Hsp90, it cannot compensate for HOP function. Downregulation of HOP/Sti1 in yeast and mammalian cell culture significantly reduces the CRAF signaling. Our data suggest that Hsp90 is recruited to CRAF in two distinct steps: first during folding/maturation via HOP and Cdc37, and later during activation mediated by HOP. Therefore, HOP is a regulator of CRAF kinase during activation of MAPK pathway and serves as a modulator of growth signaling beyond its client folding and maturation function.