The endoplasmic reticulum chaperone BiP is a closure-accelerating cochaperone of Grp94

Abstract

Hsp70 and Hsp90 chaperones provide protein quality control to the cytoplasm, endoplasmic reticulum (ER), and mitochondria. Hsp90 activity is often enhanced by cochaperones that drive conformational changes needed for ATP-dependent closure and capture of client proteins. Hsp90 activity is also enhanced when working with Hsp70, but, in this case, the underlying mechanistic explanation is poorly understood. Here we examine the ER-specific Hsp70/Hsp90 paralogs (BiP/Grp94) and discover that BiP itself acts as a cochaperone that accelerates Grp94 closure. The BiP nucleotide binding domain, which interacts with the Grp94 middle domain, is responsible for Grp94 closure acceleration. A client protein initiates a coordinated progression of steps for the BiP/Grp94 system, in which client binding to BiP causes a conformational change that enables BiP to bind to Grp94 and accelerate its ATP-dependent closure. Single-molecule fluorescence resonance energy transfer measurements show that BiP accelerates Grp94 closure by stabilizing a high-energy conformational intermediate that otherwise acts as an energetic barrier to closure. These findings provide an explanation for enhanced activity of BiP and Grp94 when working as a pair, and demonstrate the importance of a high-energy conformational state in controlling the timing of the Grp94 conformational cycle. Given the high conservation of the Hsp70/Hsp90 system, other Hsp70s may also serve dual roles as both chaperones and closure-accelerating cochaperones to their Hsp90 counterparts.

Proinsulin dependent interaction between ENPL-1/GRP94 and ASNA-1 in neurons is required to maintain insulin secretion in C. elegans

“…The ATPase activity of GRP94 is relatively low, indicating that GRP94 requires a co-chaperone to enhance its binding to the designated clients 23,45 . Recent studies have shown that BiP is a novel interacting partner of GRP94, and that BiP assists with the acceleration of the closure of GRP94 to trap the client protein 22 . However, there is a lack information on which co-chaperones might act with ENPL-1/GRP94 for proinsulin handling and processing.…”

“…GRP94 requires BiP for the accelerating the closure of its open state and trapping the proIGF2 client protein. The slow ATPase activity 23 and the slow closure of GRP94 22 suggest that it requires other chaperones to assist in its conformational changes which are required for binding its client proteins. Clients bind via the defined client binding CBD while the regions of GRP94 required for co-chaperone association are unknown.…”

“…Interestingly, the role of CNPY3 was suggested to support the loading of TLR proteins onto GRP94 indicating for the first time that an ER luminal chaperone requires help from other chaperones to fulfil its function. The other well-described chaperone of GRP94 is BiP [19][20][21][22] . GRP94 requires BiP for the accelerating the closure of its open state and trapping the proIGF2 client protein.…”

A proinsulin-dependent interaction between ENPL-1 and ASNA-1 in neurons is required to maintain insulin secretion in C. elegans

“…Interestingly, the role of CNPY3 has been suggested to support the loading of TLR proteins onto GRP94, indicating for the first time that an ER luminal chaperone requires help from other chaperones to fulfil its function. The other well-described chaperone of GRP94 is BiP ( Huang et al, 2022 ; Jansen et al, 2012 ; Jin et al, 2021 ; Sun et al, 2019 ). GRP94 requires BiP to accelerate its open state and for trapping the proIGF2 client protein.…”

“…GRP94 requires BiP to accelerate its open state and for trapping the proIGF2 client protein. The slow ATPase activity ( Frey et al, 2007 ) and the slow closure of GRP94 ( Huang et al, 2022 ) suggest that GRP94 requires other chaperones to assist in the conformational changes that are required for its binding to client proteins. Clients bind via the defined client-binding domain CBD but the regions of GRP94 required for the co-chaperone association are unknown.…”