Difference between PA700-like proteasome activator complex and the regulatory complex dissociated from the 26S proteasome implies the involvement of modulating factors in the 26S proteasome assembly

The PA700-like proteasome activator complex was highly purified from porcine erythrocytes, and its properties were compared with those of the regulatory complex disassembled from the purified 26S proteasome. The molecular mass of the PA700-like complex, which comprises 25–110-kDa subunits, was estimated to be 800 kDa by Superose 6 gel filtration. This complex showed neither ATPase activity nor peptidase activity toward Suc–Leu–Leu–Val–Tyr–MCA. Nevertheless, it was possible to make a high molecular mass complex from the purified PA700-like complex by incubating with the 20S proteasome in the presence of ATP. In contrast, the regulatory complex dissociated from the 26S proteasome did not reconstitute a larger complex under the same conditions. The subunit composition of the PA700-like complex was similar but not identical to that of the regulator complex dissociated from the 26S proteasome: the former complex had a 25-kDa subunit which is absent in the latter, whereas the latter had two or three 43-kDa subunits lacking in the former. These results indicate that the purified PA700-like proteasome activator complex is structurally and functionally distinct from the regulatory complex dissociated from the 26S proteasome, implying the involvement of modulating factors in the 26S proteasome assembly.