Catalytic components of the bovine pituitary multicatalytic proteinase complex (proteasome).

The multicatalytic proteinase complex (MPC), also called the proteasome, is a ubiquitous particle (19S) that is required for life. It is found in the cytoplasm and nucleus of all eukaryotic cells where it degrades selected cytosolic and nuclear proteins. It forms the proteolytic core of the 26S complex that represents the final step in the ubiquitin-dependent pathway of proteolysis. The MPC expresses at least five distinct proteolytic activities. Three activities preferring cleavages on the carboxyl side of neutral amino acids were described: an activity cleaving after branched chain residues, termed branched chain amino acid preferring, that is a major factor in the degradation of proteins, an activity preferring cleavages after bulky hydrophobic residues designated chymotrypsin-like, and an activity cleaving between small neutral amino acids. Activities cleaving after basic (trypsin-like) and acidic residues (peptidylglutamyl peptide-hydrolyzing) have also been described. The expression of multiple proteolytic activities with diverse specificities may provide a functional advantage that allows efficient hydrolysis of target proteins.