THE CHAPERONE COFACTOR HOP P60 INTERACTS WITH THE CYTOSOLIC CHAPERONIN-CONTAINING TCP-1 AND AFFECTS ITS NUCLEOTIDE EXCHANGE AND PROTEIN-FOLDING ACTIVITIES/

The folding of protein structures often: requires the presence of mole cular chaperones and/or chaperonin complexes. We here investigated the inhibitory effects of the chaperone cofactors Hop/p60 and Hap46. By c oimmunoprecipitation, we observed a direct interaction of the eukaryot ic chaperonin-containing TCP-1 (CCT) purified from rabbit reticulocyte lysate with Hop/p60. By contrast, Hap46 was not coprecipitated. Bindi ng of Hop/p60 to CCT is dependent on the presence of ATP or ADP and oc curs through carboxyl-terminal sequences of Hop/p60. Hop/p60 significa ntly stimulates nucleotide exchange on CCT but not its ATPase activity , while Hap46 has no effects. We used denatured firefly luciferase as a model protein and found decreased binding to CCT in the presence of Hop/p60 and ATP. This coincides with the inhibitory effect of Hop/p60 on luciferase reactivation in an assay using purified CCT in combinati on with hsc70 and hsp40. We also observed that an antibody directed ag ainst one of the subunits of CCT efficiently inhibits refolding in a s ystem which depends on crude reticulocyte lysate.