The 26S proteasome: a dynamic structure.

The proteasomal system consists of a proteolytic core, the 20S proteasome, which associates in ATP-dependent and independent reactions with endogenous regulators providing specific substrate binding sites, chaperone function and regulation of activity to the protease. The best known regulators of the 20S proteasome are the 11S and the 19S complexes. Three subunits of the 20S proteasome and the two subunits of the 11S regulator are induced by gamma-Interferon. However, there are no indications for an influence of gamma-interferon on the subunit composition of the 19S regulator and only a few data exist about the dynamics of this complex. The analysis of 19S regulator subunits from yeast mutants reveals that the ATPases appear to be stringently organized in the 26S complex, while peripheral non-ATPases, such as S5a, might serve as subunits which shuttle substrates to the enzyme. A novel non-ATPase has been cloned, sequenced and identified in a complex besides the 19S regulator, the function of which is presently unknown. The dynamic structure of the 26S proteasome is also characterized by transient associations with components such as the modulator and isopeptidases. Certain viral proteins can also be associated with components of the proteasomal system and alter enzymatic activities.