Components of the multicatalytic proteinase complex.

The multicatalytic proteinase complex is a high molecular weight nonlysosomal proteinase. Kinetic studies of the proteolytic activities of the complex have shown that there are at least three distinct types of catalytic centre, each of which has a different specificity. All of the activities can be inhibited by the serine protease inhibitor 3,4-dichloroisocoumarin. Viewed under the electron microscope, the multicatalytic proteinase purified from rat liver appears to have a hollow cylindrical structure. It is composed of many different types of subunit and on two-dimensional polyacrylamide gels gives rise to a complex pattern of about 20 spots with pI values ranging between 5 and 8.5 and molecular masses between 22 and 34 kDa. Immunoblot analysis has shown that many of the major polypeptides are antigenically distinct. However, there are some relationships between the proteinase polypeptides. For example, although N-terminal sequences of five of the polypeptides are unique, they show considerable sequence similarity suggesting that these proteins are encoded by members of the same gene family. Also, there is some cross-reactivity between certain polypeptides when blots are probed with affinity purified, subunit-specific antisera. In addition to the variety of polypeptide components of the proteinase, a small RNA species (80 nucleotides) can be found associated with the complex even after purification by chromatographic procedures.