Modulation of the Hsp70 Protein Quality Control System by Phosphorylation of Chaperones and Co‐chaperones

Abstract

The 70‐kilodalton heat shock protein (Hsp70) interacts with tetratricopeptide repeat (TPR) domains of the co‐chaperones CHIP (C‐terminus of Hsp70 Interacting Protein) and HOP (Hsp Organizing Protein) using a two‐carboxylate clamp binding mechanism. Hsp70 can only interact with one of the co‐chaperones at a time, thus modulating interactions of Hsp70 with either CHIP or HOP controls the Hsp70 protein quality control axis by shunting misfolded client proteins to either the degradative pathway or the refolding pathway. Interactions of Hsp70 with CHIP promote CHIP‐mediated degradation via the ubiquitin proteasome pathway. Interactions of Hsp70 with HOP promote protein refolding since HOP connect Hsp70 with the 90 kilodalton heat shock protein Hsp90. Differential affinity for the TPR domains of CHIP and HOP are achieved through interactions with the six residues preceding the carboxy‐terminal Asp641 of Hsp70. In particular, phosphorylation of Hsp70 Thr636 produces a dramatic swing in binding affinities that shift preferential formation of CHIP/Hsp70 complexes to preferential formation of HOP/Hsp70 complexes. Thus, phosphorylation of Hsp70 Thr636 acts as a switch that directs the output and function of cellular protein quality control. Phosphorylation of CHIP Ser20 or CHIP Ser24 also modulates the interactions between CHIP and Hsp70, providing yet another level of regulation for the Hsp70 protein quality control system. Herein, through biophysical assays and new structures of CHIP‐Hsp70 and HOP‐Hsp70 we identify the molecular mechanism that enables phosphorylation‐driven switching that directs interactions within the CHIP‐Hsp70/HOP‐Hsp70 protein quality control complexes. We also present a structure‐guided mutation of Hsp70 that bypasses Thr636 phosphorylation‐driven switching by recovering affinity of CHIP for phosphorylated Hsp70, thereby redirecting Hsp70 to preferentially bind to CHIP. This structure‐guided mutation corrects a binding defect present in wild type CHIP and provides a system for studying the effect of shifting output of the CHIP‐Hsp70/HOP‐Hsp70 protein quality control complexes toward chaperoned ubiquitination via the CHIP‐Hsp70 complex.Support or Funding InformationNSF 15‐52113This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.