HOP MODULATES HSP70 HSP90 INTERACTIONS IN PROTEIN-FOLDING/

Hop is a 60-kDa protein characterized by its ability to bind the two c haperones, hsp70 and hsp90. We have tested the function of Hop using a n assay for the refolding of denatured firefly luciferase. We show tha t Hop is involved in the process of refolding thermally denatured fire fly luciferase in rabbit reticulocyte lysate. Hop also stimulates refo lding by hsp70 and Ydj-1 in a purified refolding system. Hsp90 can als o stimulate refolding, and optimal refolding is observed in the presen ce of both Hop and hsp90. Similar stimulation was observed when Hop wa s replaced by its yeast homolog Sti1. In assays of the binding of Hop to hsp70 and hsp90, Hop preferentially forms a complex with ADP-bound hsp70, and this process is unaffected by the presence of hsp90. Hop do es not alter the ATPase activity or the rate of ADP dissociation of hs p70. Hop also appears to bind to the ADP-bound form of hsp90, blocking the ATP-dependent conversion of hsp90 to a form capable of interactin g with p23. Conversely, once p23 is bound to hsp90, Hop binding is dim inished. These results confirm that Hop provides a physical link betwe en hsp70 and hsp90 and also indicate that Hop modulates the activities of both of these chaperone proteins.