Since the introduction of isotopic tracers into biochem- many as 22 different names in the literature; many funcistry over 50 years ago, it has been known that intracellu- tions were proposed for it that were unrelated to protein lar proteins, once synthesized, are continually being turnover [4]. It was eventually named the ‘proteasome’to degraded back to their constituent amino acids. But clear indicate a particle with proteolytic function [4]. Since ideas about the degradative pathways responsible, the then, antibody studies in vitro, reconstitution, and finally proteolytic machinery involved, the mode of action of mutant studies in yeast have shown that the 20s particle the enzymes required, and the physiological significance functions in the ubiquitin-dependent pathway as the proof the process have only recently emerged. It is now teolytic core of the 26s complex [9, 10]. Formation of this known that the bulk of cell proteins are hydrolyzed by a larger structure involves an ATP-dependent association of soluble ATP-dependent system that is present in both the the 20s proteasome with a 19s (600 kDa) particle nucleus and cytosol [l]. Protein substrates are first[11,121, which appears to function as the ‘mouth’for the marked for degradation by covalent conjugation to mul- 20s particle (Fig. 2); it binds substrates, probably unfolds tiple molecules of a small protein, ubiquitin [l]. This them, and injects the polypeptides into the 20s proteaprocess involves the activation of ubiquitin by the forma- some for digestion [2, 8]. The 19s particle contains five tion of a thiol-ester at its carboxyl terminus, which is distinct ATPases, a binding site for ubiquitin chains [13], then transferred to the E-amino group on a lysine residue and at least one isopeptidase [14], which disassembles the on the protein (Fig. 1). Other ubiquitin molecules are ubiquitin chain during protein hydrolysis. Despite this processively linked to the first, forming long chains of exciting recent progress, many aspects of the function of ubiquitin on the substrate. This triggers the rapid hydrol- the 26s proteasome complex remain mysterious. For ysis of the substrate by a very large ATP-dependent pro- example, the function of ATP hydrolysis is not known. We teolytic complex, termed the 26s proteasome (for useful reviews, see [2-81). The proteolytic core of this 2000 kDa do not understand why degradation of some polypeptides (eg the enzyme ornithine decarboxylase) does not (26-30s) structure is the 20s proteasome, a 700-kDa require ubiquitination [15], nor do we know whether the particle containing multiple peptidase activities [2-8). 20s particle ever breaks down proteins by itself in vim.