Mechanism of client loading from BiP to Grp94 and its disruption by select inhibitors

Area of Science:

• Molecular Biology
• Cancer Biology
• Structural Biology

Background:

• Heat shock protein 90 (Hsp90) is a crucial molecular chaperone and a validated cancer drug target.
• Numerous ATP-competitive Hsp90 inhibitors are in clinical trials for cancer treatment.
• The precise mechanisms by which Hsp90 inhibitors influence client protein transfer and loading remain incompletely understood.

Purpose of the Study:

• To investigate how Hsp90 inhibitors affect the client protein delivery, loading, and trapping steps.
• To elucidate the role of the Hsp70 chaperone BiP in facilitating client loading onto Hsp90 paralog Grp94.
• To identify structural determinants governing inhibitor effects on Hsp90 client loading.

Main Methods:

• Comparative analysis of the ER-resident BiP/Grp94 chaperone system (Hsp70/Hsp90 paralogs) in the presence of different Hsp90 inhibitors.
• Biochemical assays to examine BiP-mediated conformational changes in Grp94.
• Structural studies to understand inhibitor interactions and their impact on ternary complex formation.

Main Results:

• Specific Hsp90 inhibitors differentially modulate BiP's ability to induce the client-loading conformation of Grp94.
• Certain inhibitors block BiP-driven conformational changes, leading to destabilization of BiP/client/Grp94 ternary complexes.
• A structural basis was identified for selective inhibition of client loading by a subset of Hsp90 inhibitors.

Conclusions:

• BiP actively drives Grp94 into a client-loading-competent state through a mechanism conserved between ER and cytosolic Hsp70/Hsp90 systems.
• Understanding these client loading dynamics reveals how specific inhibitors disrupt Hsp90 function.
• These findings provide a foundation for developing novel, rationally designed Hsp90-targeting cancer therapies.