The base of the proteasome regulatory particle exhibits chaperone-like activity

Protein substrates of the proteasome must apparently be unfolded and transl ocated through a narrow channel to gain access to the proteolytic active si tes of the enzyme. Protein folding in vivo is mediated by molecular chapero nes. Here, to test for chaperone activity of the proteasome, we assay the r eactivation of denatured citrate synthase, Both human and yeast proteasomes stimulate the recovery of the native structure of citrate synthase, We map this chaperone-like activity to the base of the regulatory particle of the proteasome, that is, to the ATPase-containing assembly located at the subs trate-entry ports of the channel. Denatured but not native citrate synthase is bound by the base complex. Ubiquitination of citrate synthase is not re quired for its binding or refolding by the base complex of the proteasome, These data suggest a model in which ubiquitin-protein conjugates are initia lly tethered to the proteasome by specific recognition of their ubiquitin c hains; this step is followed by a nonspecific interaction between the base and the target protein, which promotes substrate unfolding and translocatio n.