ABSTRACT

I t has been nearly two decades since Hershko and colleagues elucidated the pathway that conjugates ubiquitin (Ub) to intracellular proteins and selects them for destruction.1'4 A second energy-dependent step in the degradation of Ub-tagged substrates was identified in 1984.5 6 This additional ATPconsuming reaction was eventually attributed to a large 26 S protease discovered in 1986 by Hough et a l.7 Purification of the enzyme capable of degrading polyubiquitinatedlysozyme was reported the next year.8 In the presence of ATP and M gC l2 the 26 S proteasome can be reconstituted from three conjugate-degrading factors, CF-1, CF-2, and CF-3, to form a large complex similar to the one described by Hough et al.9 Several groups < have demonstrated that the multicatalytic protease (MCP) or proteasome is identical to < CF-3 and constitutes the proteolytic core of ; the 26 S enzyme.10'12 Unlike the larger 26 S enzyme, however, the proteasome is essentially ] an energy-independent peptidase unable to degrade ubiquitinated proteins.8 Proteasomes are cylindrical particles composed of four stacked heptameric rings and the crystal structures of proteasomes from the archeabacterium T a cid oph ilum and the yeast S. cerev isia e have been solved.13,14 Upon ] examination of the crystal structures, one 1 immediately recognizes the need for a protein ‘ to be in an unfolded state to gain access to the (

central chamber containing the catalytic sites. However, unlike the archeabacterial pro­ teasomes, yeast proteasomes are sealed at the ends of the cylinder thus blocking access to the β-subunit active sites.14 It is therefore obvious that other regulatory molecules must enable the proteasome to perform specific biological functions, such as proteolysis of ub iqu itin ated proteins (Fig. 7 .1 ) . The minimum requirements for such a regulator would be the ability to recognize substrate proteins and to translocate the substrates into the proteasome s inner chamber where peptide bond hydrolysis occurs. This review will focus on our current understanding of the structure and function of the mammalian version of one of these regulators, a ^700-1000 kDa multi­ protein complex variably termed the regulatory complex (RC), the ball, ATPase complex, 19 S cap complex, μ particle, or PA700.12,15-19 For reasons oudined below, we will use the term regulatory complex throughout this essay.