The multicatalytic and 26 S proteases.

Abstract

Two large, multisubunit proteases are abundant constituents in the cytosol/nuclear compartments of eucaryotic cells. Among many names proposed for the smaller enzyme, multicatalytic protease (MCP)l or 20 S proteasome is most commonly used. The larger enzyme, discovered by its ability to degrade ubiquitin (Ub)-lysozyme conjugates in an ATP-dependent reaction, is called the 26 S protease or 26 S proteasome. As depicted by the models in Fig. 1, the two proteases share subunits. They are thought to play roles in a diverse set of processes including cell cycle traverse, antigen presentation, and selective intracellular proteolysis. Here, we review MCP and the 26 S enzyme with emphasis on their structure, proteolytic mechanisms, and interrelationships. Because limited space precludes lengthy discussion on any topic, we have not been comprehensive. Rather, we attempt to present reasonable hypotheses based on recent findings. There are longer reviews on MCP (1, 2), its possible role in antigen processing (3), and on various aspects of ubiquitin-dependent proteolysis (4). In addition, Maurizi (5) provides a superb discussion on the related Escherichia coli ATP-dependent proteases, Lon and Clp (5).