Proteasomes: protein degradation machines of the cell.

20 S proteasomes are cylinder-shaped protein complexes that play an important role in intracellular protein degradation in eukaryotes and certain archaebacteria. Although 20 S proteasomes were first described many years ago, it has been discovered only recently that these particles can assemble with other protein complexes, presumably of regulatory function. One of the macromolecular assemblies formed, the 26 S proteasome complex, functions as an ATP-dependent protease in the ubiquitin pathway, which has been implicated in the degradation of abnormal proteins, degradation of short-lived regulatory proteins and antigen presentation.