Structural Studies on the Co-chaperone Hop and Its Complexes with Hsp90

The tetratricopeptide repeat domain (TPR)-containing co-chaperone Hsp-organising protein (Hop) plays a critical role in mediating interactions between Heat Shock Protein (Hsp)70 and Hsp90 as part of the cellular assembly machine. It also modulates the ATPase activity of both Hsp70 and Hsp90, thus facilitating client protein transfer between the two. Despite structural work on the individual domains of Hop, no structure for the full-length protein exists, nor is it clear exactly how Hop interacts with Hsp90, although it is known that its primary binding site is the C-terminal MEEVD motif. Here, we have undertaken a biophysical analysis of the structure and binding of Hop to Hsp90 using a variety of truncation mutants of both Hop and Hsp90, in addition to mutants of Hsp90 that are thought to modulate the conformation, in particular the N-terminal dimerisation of the chaperone. The results establish that whilst the primary binding site of Hop is the C-terminal MEEVD peptide of Hsp90, binding also occurs at additional sites in the C-terminal and middle domain. In contrast, we show that another TPR-containing co-chaperone, CyP40, binds solely to the C-terminus of Hsp90.