FTIR Study Reveal Intrinsically Disordered Nature of Heat Shock Protein 90

Heat shock protein 90 (Hsp90) is a highly conserved chaperone protein that enables the proper folding of a large number of structurally diverse proteins (a.k.a., clients) in the crowded cytosolic environment and plays a key role in regulating the heat shock response. An outstanding question is how Hsp90 accommodates the structural diversity of a large cohort of client proteins? We report ATR FTIR study on structural properties of Hsp90 C-terminal domain (CTD) and their temperature dependences. Our data reveal that within a narrow temperature window, from 35o to 45 oC around physiological temperatures, Hsp90CTD exhibits significant increases in protein aggregation and increases in unordered structures. Binding of Hsp90 inhibitor Clorobiocin elevates the transition temperature of protein aggregation from 38 °C (a physiological temperature) to 65 °C (far above the physiological temperatures). In striking contrast, the Hsp90 N-terminal domain and middle domain (NTMD) demonstrates no protein aggregation from 20° to 90 °C. Despite the intrinsically disordered nature of Hsp90CTD, it retains a protected hydrophobic core at 40 °C. We introduce a hydrophobic interlock (HPI) model to account for the functional capability of intrinsically disordered Hsp90. Implications of these results and the HPI model will be discussed in the light of the structural dynamics and client diversity of Hsp90.