Molecular chaperone activity of the 20S proteasome : Inhibitory activity of the substrate protein aggregation and unflodase activity

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Cellular proteases and molecular chaperone proteins play important roles in the intracellular protein catabolism. Among the proteases, a multi-functional 20S proteasome, which is composed of 28 low molecular mass subunits arranged in a stack of four rings, each containing seven different α- and β-subunits, plays a central role in the degradation of unfolded and/or misfolded proteins in the cytosol in cooperation with molecular chaperones. We found that the C5 in theβ-type and the C8 in theα-type subunits of the 20S proteasome reveal the activities of ATP hydrolysis and ADP-ATP exchange, which are similar to the activities of molecular chaperone and essential to the association and dissociation of substrate proteins and peptides. Furthermore, 20S proteasome reveals a chaperone-like activity, an inhibition of the aggregation of heat-denatured proteins, in an ATP-independent manner and degradation of unfolded protein in an ATP-dependent manner. These results suggest the activities of ATP hydrolysis and ATP-ADP exchange of the 20S proteasome play a role in translocation of unfolded and/or misfolded proteins into the catalytic cavity of the inside of ring-shaped 20S proteasome but not play a role in inhibition of the aggregation of heat-denatured proteins.