The essential co-chaperone Sgt1 regulates client dwell time in the Hsp90 chaperone cycle

The Hsp90 molecular chaperone system is regulated by numerous co-chaperones that modulate its function. In Saccharomyces cerevisiae, most of these cofactors can be deleted without affecting viability. Of the three essential ones, only the function of Sgt1 has remained enigmatic. Our in vivo and in vitro experiments define key structural elements and determine the essential function of Sgt1 in the chaperoning of client proteins. We demonstrate that yeast Sgt1 adopts a unique binding mode, engaging primarily with the middle domain of Hsp90. Through simultaneous interaction with both Hsp90 and client proteins, Sgt1 enhances client maturation efficiency. Specifically, Sgt1 stabilizes Hsp90-client complexes and prevents their dissociation by the co-chaperone Aha1. Our findings reveal a previously unrecognized layer of Hsp90 regulation, highlighting Sgt1 as a critical modulator of chaperone cycle progression.