Beyond a Passive Tether: Structural Insights into the Disordered Tail of Hsp90

Area of Science:

• Molecular Biology
• Structural Biology
• Biophysics

Background:

• Heat shock protein 90 (Hsp90) is a crucial molecular chaperone involved in protein folding and stability.
• The C-terminal domain (CTD) of Hsp90 facilitates dimerization and cochaperone binding via the MEEVD motif on its disordered tail.
• The precise structural role of the Hsp90 CTD tail remains incompletely understood.

Purpose of the Study:

• To investigate the conformational dynamics of the yeast Hsp90 (yHsp90) CTD tail.
• To examine how the binding of the cochlearin Cpr6 affects the CTD tail conformation.
• To elucidate the structural contribution of the CTD tail to Hsp90 architecture.

Main Methods:

• Site-directed spin labeling using nitroxide and Gd(III) labels.
• Double electron-electron resonance (DEER) and Electron Paramagnetic Resonance (EPR) spectroscopy.
• Analysis of full-length yHsp90, isolated CTD, and tail-truncated variants.

Main Results:

• CTD tails in isolated CTD (IsoC) and full-length (FL) yHsp90 exhibit distinct conformational ensembles, with differences attributed to intramolecular interactions in FL.
• Cochlearin Cpr6 binding disrupts these intramolecular contacts, diminishing the conformational differences between IsoC and FL.
• The CTD tail stabilizes a unique CTD conformation in IsoC, which is absent in FL, reduced upon tail truncation, and diminished by Cpr6 binding.

Conclusions:

• The intrinsically disordered CTD tail of Hsp90 is not merely a passive element but an active modulator of Hsp90 conformation.
• The tail's conformational ensemble and CTD architecture are dynamically regulated by intramolecular interactions and cochlearin binding.
• These findings highlight the functional significance of the CTD tail in the Hsp90 chaperone cycle.