Multiple domains of the co-chaperone Hop are important for Hsp70 binding

The Hop/Sti1 co-chaperone binds to both Hsp70 and Hsp90. Biochemical and co-crystallographic studies have suggested that the EEVD-containing C terminus of Hsp70 or Hsp90 binds specifically to one of the Hop tetratricopeptide repeat domains, TPR1 or TPR2a, respectively. Mutational analyses of Hsp70 and Hop were undertaken to better characterize interactions between the C terminus of Hsp70 and Hop domains. Surprisingly, truncation of EEVD plus as many as 34 additional amino acids from the Hsp70 C terminus did not reduce the ability of Hsp70 mutants to co-immunoprecipitate with Hop, although further truncation eliminated Hop binding. Hop point mutations targeting a carboxylate clamp position in TPR1 disrupted Hsp70 binding, as was expected; however, similar point mutations in TPR2a or TPR2b also inhibited Hsp70 binding in some settings. Using a yeast-based in vivo assay for Hop function, wild type Hop and TPR2b mutants could fully complement deletion of Sti1p; TPR1 and TPR2a point mutants could partially restore activity. Conformations of Hop and Hop mutants were probed by limited proteolysis. The TPR1 mutant digested in a similar manner to wild type; however, TPR2a and TPR2b mutants each displayed greater resistance to chymotryptic digestion. All point mutants retained an ability to dimerize, and none appeared to be grossly misfolded. These results raise questions about current models for Hop/Hsp70 interaction.