Dual Roles for Yeast Sti1/Hop in Regulating the Hsp90 Chaperone Cycle

Abstract

The Hsp90 chaperone is regulated by many cochaperones that tune its activities, but how they act to coordinate various steps in the reaction cycle is unclear. The primary role of Hsp70/Hsp90 cochaperone Sti1 (Hop in mammals) is to bridge Hsp70 and Hsp90 to facilitate client transfer. Sti1 is not essential, so Hsp90 can interact with Hsp70 without Sti1. Nevertheless, many Hsp90 mutations make Sti1 necessary. We noted that Sti1-dependent mutations cluster in regions proximal to N-terminal domains (SdN) or C-terminal domains (SdC), which are known to be important for interaction with Hsp70 or clients, respectively. To uncover mechanistic details of Sti1-Hsp90 cooperation, we identified intramolecular suppressors of the Hsp90 mutants and assessed their physical, functional, and genetic interactions with Hsp70, Sti1, and other cochaperones. Our findings suggest Hsp90 SdN and SdC mutants depend on the same interaction with Sti1, but for different reasons. Sti1 promoted an essential Hsp70 interaction in the SdN region and supported SdC-region function by establishing an Hsp90 conformation crucial for capturing clients and progressing through the reaction cycle. We find the Hsp70 interaction and relationship with Sti1/Hop is conserved in the human Hsp90 system. Our work consolidates and clarifies much structural, biochemical, and computational data to define roles of Sti1/Hop in coordinating Hsp70 binding and client transfer with progression of the Hsp90 reaction cycle.

Mutations in the Hsp90 N Domain Identify a Site that Controls Dimer Opening and Expand Human Hsp90α Function in Yeast

“…We discovered that hHsp90α, unlike hHsp90β, was unable to complement essential yeast Hsp90 functions in our strains ( Figure 1(a) ) [ 24 ]. This inability is in contrast to previous studies from two groups that reported weak complementation by hHsp90α compared to hHsp90β [ 20 , 21 ].…”

“…Recently, we showed that non-lethal mutations in Hsp82 that become lethal when the co-chaperone Sti1 (Hop in humans) is absent cluster into distinct regions that define two classes of Sti1-dependent mutations [ 24 ]. One class (SdN) is at the site of Hsp70 interaction in the M domain [ 33 ], and the other (SdC) is at the junction of the M and C domains.…”

“…One class (SdN) is at the site of Hsp70 interaction in the M domain [ 33 ], and the other (SdC) is at the junction of the M and C domains. The SdN mutants require Sti1 for efficient interaction with Hsp70 and the SdC mutants require a separate function of Sti1 to facilitate Hsp90 dimer closing [ 24 ]. Unlike Hsp82, wild-type hHsp90β cannot support yeast growth when Sti1 is absent.…”

“…This Sti1 dependency is rescued by the substitution E414R, which is analogous to E402R of Hsp82 that enhances Hsp70 binding. hHsp90β therefore behaves like an SdN mutant that needs Sti1 to interact effectively with Hsp70 in yeast [ 24 ].…”

Hsp90 and Hsp70 chaperones: Collaborators in protein remodeling

“…Additional studies show that human mitochondrial Hsp90, TRAP1, interacts with mitochondrial Hsp70, mortalin, in BLI assays in vitro (103), consistent with the lack of a Hop homolog in mitochondria. In addition, cytoplasmic human Hsp90␤ is seen with Hsc70 in pulldown assays (104).…”