Abstract
BiP and Grp94, the endoplasmic reticulum (ER) paralogs of the highly conserved Hsp70 and Hsp90 chaperone families, work together in client protein folding and maturation. During their working cycle, BiP can directly bind to Grp94 and transfer a client protein to Grp94. BiP accelerates ATP-dependent closure of the Grp94 dimer by stabilizing a high-energy conformational intermediate of Grp94 (C’ state). However, the structural basis of this acceleration is unknown. A newly published cytosolic Hsp70/Hsp90 complex cryo-EM structure suggests a structure of the Grp94 C’ state, which shows a novel “interface II” between the Hsp70 nucleotide-binding domain (NBD) and Hsp90 N-terminal domain (NTD). Here I find that the interface II plays two important roles for BiP and Grp94. First, BiP accelerates Grp94 closure by stabilizing the high-energy C’ state via contacts at interface II. Second, a BiP/Grp94/client ternary complex is stabilized by interface II during client transfer. Surprisingly, interface II mutations on BiP greatly increase the affinity of BiP for clients under ATP conditions but not under ADP conditions, which opens a new possibility for understanding the mechanism by which BiP achieves high affinity for client proteins. Our results show that interface II is conserved between the ER and the cytosolic Hsp70/Hsp90 systems, which implies that their client transfer mechanism is conserved.