Structure and Function of the 20S Proteasome and of Its Regulatory Complexes

Proteasomes are ubiquitous, multisubunit proteases with highly conserved structures (for review, see Lupas et al. 1993). The 26S proteasome, an ATP-dependent enzyme of about 2000 kD, is the central protease of the ubiquitin- dependent pathway of protein degradation (for review, see Hochstrasser 1995). Detected so far only in eukaryotes, it plays a central role in a variety of cellular processes, including cell cycle (Glotzer et al. 1991; Ghislain et al. 1993; Gordon et al. 1993), transcriptional regulation (Dubiel et al. 1993, 1994; DeMartino et al. 1994), and antigen presentation (for review, see Gaczynska et al. 1993). The core of the 26S complex is formed by the 20S proteasome, an ATP-independent, barrel-shaped protease of about 700 kD, which is found in all three kingdoms of life.