Molecular cloning and expression of a 26 S protease subunit enriched in dileucine repeats

The 26 S protease is a multisubunit enzyme required for ubiquitin-dependent proteolysis. Recently, we identified a 50-kDa subunit (S5) of this enzyme that binds ubiquitin polymers (Deveraux, Q., Ustrell, V., Pickart, C., and Rechsteiner, M. (1994) J. Biol. Chem. 269, 7059-7061). We have now isolated, sequenced, and expressed a cDNA encoding a novel 50-kDa subunit of the 26 S protease. The recombinant protein does not bind ubiquitin polymers. Two-dimensional electrophoresis reveals that two subunits of the 26 S protease have apparent molecular masses of 50 kDa. Antibodies specific for the recombinant protein recognize the more basic of the two subunits (S5b), whereas the more acidic subunit (S5a) binds ubiquitin chains. Thus, the 26 S protease contains at least two distinct subunits with apparent molecular masses of 50 kDa.