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-depe ndent proteolysis. Recently, we identified a 50-kDa subunit (S5) of th is enzyme that binds ubiquitin polymers (Deveraux, Q., Ustrell, V., Pi ckart, 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 5 0 kDa. Antibodies specific for the recombinant protein recognize the m ore 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.