BOVINE SPLEEN MULTICATALYTIC PROTEINASE COMPLEX (PROTEASOME) - REPLACEMENT OF X-SUBUNIT, Y-SUBUNIT, AND Z-SUBUNIT BY LMP7, LMP2, AND MECL1 AND CHANGES IN PROPERTIES AND SPECIFICITY

Amino acid sequencing of subunits of the multicatalytic proteinase com plex (MPC) isolated from bovine spleen showed an almost complete repla cement of the X, Y, and Z subunits, constitutively expressed in most t issues, by the interferon-gamma-inducible LMP7, LMP2, and MECL1 subuni ts. A comparison with the pituitary MPC found a decreased chymotrypsin -like activity, a depressed peptidylglutamyl-peptide hydrolyzing activ ity, and a highly active component with properties similar to, but not identical with, that of the pituitary branched chain amino acid prefe rring (BrAAP) component. Unlike the pituitary BrAAP component, that of the spleen MPC exhibited a greatly decreased K-m, a highly increased catalytic efficiency (k(cat)), and a 80-180 times greater specificity constant (k(cat)/K-m) toward substrates with either branched chain or aromatic amino acid residues in the P-1 position. Also, unlike the pit uitary BrAAP component, that of the spleen was sensitive to inactivati on by 3,4-dichloroisocoumarin and sensitive to inhibition by peptidyl- aldehydes with either phenylalaninal or leucinal residues. Several phe nylalaninal peptidyl-aldehydes were identified which selectively inhib ited components of the spleen but not of the pituitary MPC. Two of the inhibitors are dipeptidyl-aldehydes, two others are tetrapeptidyl-ald ehydes with a Pro residue in the P-3 position. The possibility is disc ussed that the properties and specificity of the spleen MPC are a cons equence of the presence of the interferon-gamma-inducible subunits.