Characterization of mouse proteasome subunit MC3 and identification of proteasome subtypes with different cleavage characteristics. Proteasome subunits, proteasome subpopulations.

We have isolated and characterized a cDNA encoding the mouse proteasome subunit MC3 and identified four proteasome subtypes which differ in their peptide-hydrolyzing and polypeptide-cleavage properties. Immunoblotting data show that the 25-kD MC3 subunit is a constitutive proteasome subunit which exists in several isoforms. In addition, by immunoprecipitation of proteasomes with AbMC3, a subset of enzyme complexes could be recognized which differ in their relative subunit composition from the bulk of proteasomes. Using DEAE-column chromatography we identified three different proteasome subtypes in sol-80 mouse liver extracts and, by Trition X-100 extraction, a distinct membrane-bound subtype. The four proteasome subtypes are shown to differ in their trypsin- and chymotrypsin-like hydrolyzing activities as well as in their ability to cleave a 25mer polypeptide substrate derived from the MCMV IE pp89. Our data indicate that the enzymatic properties observed for the total proteasome population may be the summary of cleavage properties of different types of proteasome complexes.