Subtypes of 20S proteasomes from skeletal muscle.

20S proteasomes from tissues and cells are a mixture of several subtypes. From rat skeletal muscle we have tentatively separated six different subtypes of 20S proteasomes purified from rat skeletal muscle by high-resolution anion exchange chromatography. Immunoblot analysis using antibodies to the beta-subunits LMP2, LMP7 and their constitutive counterparts delta and MB1 revealed that two of the three major subtypes (subtypes I and II) are constitutive proteasomes, whereas two of the three minor subtypes belong to the subpopulation of immuno-proteasomes. Subtype III and IV are intermediate-type proteasomes. Enzymological characterisation of the six subtypes revealed clearly different V(max) values for hydrolysis of fluorogenic peptide substrates as well as significantly different activities measured with a 25-mer polypeptide of the murine cytomegalovirus IE pp89 protein as substrate. Our data show that the properties of 20S proteasomes isolated from a given tissue or cells are always the average of the properties of the whole set of proteasome subtypes.