PA700, an ATP-dependent activator of the 20 S proteasome, is an ATPase containing multiple members of a nucleotide-binding protein family

PA700 is a 700,000-dalton multisubunit protein that activates multiple proteolytic activities of the 20 S proteasome by a mechanism dependent upon ATP hydrolysis (Ma, C.-P., Vu, J. H., Proske, R. J., Slaughter, C. A., and DeMartino, G. N. (1994) J. Biol. Chem. 269, 3539-3547). In order to determine the identities of and structural relationships among the subunits of PA700, individual PA700 subunits were isolated by a combination of reverse phase high performance liquid chromatography (HPLC) and SDS-polyacrylamide gel electrophoresis. Seven of the 16 subunits of PA700 so isolated were subjected to solid phase protease digestion followed by reverse phase HPLC. Selected peptides from each protein were sequenced by automated Edman degradation. Comparison of the resulting amino acid sequences with those in current data bases indicated that three of the subunits represented novel proteins, whereas four subunits were homologous to previously described proteins. Three subunits of the latter group were, in turn, homologous to one another and are members of a large family of proteins containing a consensus sequence for ATP binding. Purified PA700 demonstrated ATPase activity. Treatment of PA700 with alkylating agents, such as N-ethylmaleimide, inhibited with similar kinetics both proteasome activation and ATPase activity, suggesting that these two activities are functionally linked. Thus, PA700 is composed of multiple members of a protein family that may function in the ATP-dependent regulation of proteasome activity.