Bipartite Role of Hsp90 Keeps CRAF Kinase Poised for Activation [Protein Structure and Folding]

October 5th, 2016 by Mitra, S., Ghosh, B., Gayen, N., Roy, J., Mandal, A. K.

CRAF kinase maintains cell viability, growth and proliferation by participating in MAPK pathway. Unlike BRAF, CRAF requires continuous chaperoning by Hsp90 to retain MAPK signaling. But, the reason behind the continuous association of Hsp90 with CRAF is still elusive. In this study, we have identified the bipartite role of Hsp90 in chaperoning CRAF kinase. Hsp90 facilitates Ser-621 phosphorylation of CRAF and prevents the kinase from degradation. Co-chaperone Cdc37 assists in this phosphorylation event. However, after folding the stability of the kinase becomes insensitive to Hsp90 inhibition, although the physical association between Hsp90 and CRAF remains intact. We observe that over-expression of Hsp90 stimulates MAPK signaling by activating CRAF. The interaction between Hsp90 and CRAF is substantially increased under elevated level of cellular Hsp90 and in presence of either active Ras (RasV12) or EGF. Surprisingly, enhanced binding of Hsp90 to CRAF occurs prior to the Ras-CRAF association and facilitates actin recruitment to CRAF for efficient Ras-CRAF interaction, which is independent of Hsp90s ATPase activity. However, monomeric CRAF (CRAF R401H) shows abrogated interaction with both Hsp90 and actin, thereby affecting Hsp90-dependent CRAF activation. This finding suggests that stringent assemblage of Hsp90 keeps CRAF kinase equipped for participating in MAPK pathway. Thus, the role of Hsp90 in CRAF maturation and activation acts as a limiting factor to maintain the function of a strong client like CRAF kinase.