The proteasome and its regulators

The proteasome, a multisubunit high molecular weight complex, is involved in lysosomal degradation. A number of endogenous cellular proteins that modulate proteasome function have been identified, such as the regulatory complex of the 26S proteinase and PA28. The 26S proteinase has been implicated in both the ubiquitin-dependent and - independent pathway of protein degradation, while PA28, along with proteasomes containing the -IFN inducible subunits LMP2 and LMP7, has been implicated in antigen presentation. In this study 26S protinease has been purified to homogeneity from rat liver and subunit specific antibodies produced. A comparison of the 20S and 26S proteasomes showed significant differences in the rate of peptide substrate hydrolysis. Mass spectrometry and immunoblot analysis have identified many of the 26S proteinase subunits separated by two dimensional PAGE. Using electron microscopy subunits S4 and S12 have been found to be situated on the ends of the 26S proteinase.;The regulatory subunits S3, P45, S10 and S12 have been found to exist predominantly as part of the 26S proteinase in rat liver, whereas TBP1 was found to exist freely in the cell and as part of a complex other than 26S proteinase. Subunit specific antibodies have been used to study the levels of 26S proteinase in various rat tissues, the lowest level being found in muscle and the highest in thymus. Localisation studies suggest that subcomplexes of the 26S proteinase exist. These differences may reflect different functions of the complex. Although proteasomes have been found in the nucleus and the cytoplasm, the LMP2 and LMP7 containing proteasomes were found to be associated with the ER further implicating them in antigen presentation. PA28 was found to be localised in the cytoplasm.