Reconstitution of proteasome activator PA28 from isolated subunits: optimal activity is associated with an alpha,beta-heteromultimer

PA28, a 200 kDa activator of 20S proteasomes, was purified from human placenta and was gel electrophoretically resolved into two different subunits, alpha and beta. In reconstitution experiments, alpha-subunits alone were found to re-associate forming homooligomers with an M(r) of about 200 kDa, which elicit a stimulatory effect on proteasomal peptide-hydrolyzing activity, albeit at a moderate level. Under the same conditions, isolated beta-subunits were neither found to associate nor did they display stimulatory activity. Significantly, when both alpha- and beta-subunits were present in the reconstitution assay, heteromultimers formed, concomitant with a marked increase in stimulatory activity when compared with that of alpha-homooligomers. The reconstituted PA28alpha,beta protein is indistinguishable from purified PA28 by several criteria: it displays the same molecular mass, shows the same abundance of alpha- and beta-subunits and has a similar stimulatory activity toward 20S proteasomes. These results indicate that optimal PA28 activity is associated with a heteromultimeric structure which contains the alpha- and beta-subunits in fixed stoichiometry, most likely as an alpha3beta3-heterohexamer.