Large- and small-scale purification of mammalian 26S proteasomes.

The 26S proteasome is an ATP-dependent protease known to collaborate with ubiquitin, whose polymerization acts as a marker for regulated and enforced destruction of unnecessary proteins in eukaryotic cells. It is an unusually large multi-subunit protein complex, consisting of a central catalytic machine (called the 20S proteasome or CP/core particle) and two terminal regulatory subcomplexes, termed PA700 or RP/regulatory particle, that are attached to both ends of the central portion in opposite orientations to form an enzymatically active proteasome. To date, proteolysis driven by the ubiquitin-proteasome system has been shown to be involved in a diverse array of biologically important processes, such as the cell cycle, immune response, signaling cascades, and developmental programs; and the field continues to expand rapidly. Whereas the proteasome complex has been highly conserved during evolution because of its fundamental roles in cells, it has also acquired considerable diversity in multicellular organisms, particularly in mammals, such as immunoproteasomes, PA28, S5b, and various alternative splicing forms of S5a (Rpm 10). However, the details of the ultimate pathophysiological roles of mammalian proteasomes have remained elusive. This article focuses on methods for assay and purification of 26S proteasomes from mammalian cells and tissues.