Domain:domain interactions within Hop, the Hsp70/Hsp90 organizing protein, are required for protein stability and structure.

The major heat shock protein (Hsp) chaperones Hsp70 and Hsp90 both bind the co-chaperone Hop (Hsp70/Hsp90 organizing protein), which coordinates Hsp actions in folding protein substrates. Hop contains three tetratricopeptide repeat (TPR) domains that have binding sites for the conserved EEVD C termini of Hsp70 and Hsp90. Crystallographic studies have shown that EEVD interacts with positively charged amino acids in Hop TPR-binding pockets (called carboxylate clamps), and point mutations of these carboxylate clamp positions can disrupt Hsp binding. In this report, we use circular dichroism to assess the effects of point mutations and Hsp70/Hsp90 peptide binding on Hop conformation. Our results show that Hop global conformation is destabilized by single point mutations in carboxylate clamp positions at pH 5, while the structure of individual TPR domains is unaffected. Binding of peptides corresponding to the C termini of Hsp70 and Hsp90 alters the global conformation of wild-type Hop, whereas peptide binding does not alter conformation of individual TPR domains. These results provide biophysical evidence that Hop-binding pockets are directly involved with domain:domain interactions, both influencing Hop global conformation and Hsp binding, and contributing to proper coordination of Hsp70 and Hsp90 interactions with protein substrates.