The 20S Proteasome: Subunits and Functions

Publisher Summary This chapter discusses the structural and functional aspects of eukaryotic 20S proteasomes. Proteasomes are present in all eukaryotic cells examined to date. Proteasome-like complexes with similar structural properties to those of eukaryotes were recently found in some archaebacterial and eubacterial cells, such as Thermoplasma acidophilum and Rodococcus, respectively, although their exact subunit compositions differ considerably from each other. This suggests that proteasomes are present in all living cells. Proteasomes have been implicated as major cytosolic proteinase complexes responsible for the nonlysosomal proteolytic pathway. They are present as both 20S and 26S forms in cells, although the latter form has yet been identified only in eukaryotic cells. The 20S proteasome is a multicatalytic proteinase, which is the catalytic unit of the 26S proteasome, the eukaryotic adenosine triphosphate (ATP)-dependent protease complex responsible for degrading a variety of cellular proteins that have acquired a specific degradation signal(s) such as a multiubiquitin chain. By electron microscopy, the 20S proteasome appears to be a cylindrical particle with a dimeric structure of two distinct rings.