HSP90-cochaperone complexes preferentially stabilize intermediate misfolded sta…

The HSP90 chaperone system, comprising HSP90AA1 and HSP90AB1 in complex with their cochaperones STIP1 (Stress-Induced Phosphoprotein 1) and AHSA1 (AHA1 activator of HSP90 ATPase), operates through a distinct mechanism that stabilizes intermediate misfolded conformations rather than directly recognizing exposed amyloidogenic segments. This alternative quality control pathway targets proteins in pre-amyloidogenic states where native structure is compromised but β-sheet rich amyloid cores have not yet formed. HSP90’s unique ATP-driven conformational cycle creates a molecular clamp that encapsulates partially misfolded substrates, preventing their progression toward aggregation-competent states. STIP1 functions as a critical adaptor protein that bridges HSP70 and HSP90 systems, transferring substrates from initial HSP70-mediated recognition to HSP90-dependent stabilization of salvageable conformers. AHSA1 accelerates HSP90’s ATPase activity and prolongs the closed, substrate-encapsulating conformation, effectively quarantining misfolded proteins in a kinetically stable intermediate state. This mechanism explains how cells can maintain pools of stress-damaged proteins in non-toxic conformations during proteostatic stress, preventing both aggregation and premature degradation. The HSP90 system shows particular selectivity for substrates with disrupted tertiary structure but intact secondary structure elements, representing a complementary recognition code to HSP70’s preference for exposed hydrophobic segments. This temporal segregation of chaperone function—HSP90 acting on earlier misfolding intermediates while HSP70 targets later amyloidogenic states—provides a multi-tiered defense against protein aggregation diseases.