INTERFERENCE BETWEEN PROTEINS HAP46 AND HOP P60, WHICH BIND TO DIFFERENT DOMAINS OF THE MOLECULAR CHAPERONE HSP70/HSC70/

Several structurally divergent proteins associate with molecular chape rones of the 70-kDa heat shock protein (hsp70) family and modulate the ir activities. We investigated the cofactors Hap46 and Hop/p60 and the effects of their binding to mammalian hsp70 and the cognate form hsc7 0, Hap46 associates with the aminoterminal ATP binding domain and stim ulates ATP binding two to threefold but inhibits binding of misfolded protein substrate to hsc70 and reactivation of thermally denatured luc iferase in an hsc70-dependent refolding system. By contrast, Hop/p60 i nteracts with a portion of the carboxy terminal domain of hsp70s, whic h is distinct from that involved in the binding of misfolded proteins. Thus, Hop/p60 and substrate proteins can form ternary complexes with hsc70, Hop/p60 exerts no effect on ATP and substrate binding but never theless interferes with protein refolding, Even though there is no dir ect interaction between these accessory proteins, Hap46 inhibits the b inding of Hop/p60 to hsc70 but Hop/p60 does not inhibit the binding of Hap46 to hsc70, As judged from respective deletions, the amino-termin al portions of Hap46 and Hop/p60 are involved in this interference. Th ese data suggest steric hindrance between Hap46 and Hop/p60 during int eraction with distantly located binding sites on hsp70s, Thus, not onl y do the major domains of hsp70 chaperones communicate with each other , but cofactors interacting with these domains affect each other as we ll.