Non-proteasomal Proteins

The exact cellular process by which the 26S proteasome is assembled remains unknown, but the best evidence suggests that it results from binding of independently assembled 20S proteasome and PA700/19S RP [76]. There is considerable, but incomplete, information about the assembly of the 20S proteasome [77, 78], whereas very little is known about the assembly of PA700. Formation of 26S proteasome from purified 20S proteasome and PA700 can be achieved in vitro by an 

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The 26S proteasome also degrades non-ubiquitylated proteins [71]. The short-lived enzyme ornithine decarboxylase (ODC) and the cell-cycle regulator p21Cip provide well documented examples of ubiquitin-independent proteolysis by the 26S en-... 

Proteasome inhibitors have been instrumental in identifying numerous protein substrates and in elucidating the importance of the proteasome/ubiquitin pathway in many biological processes. Initially, non-specific cell-penetrating peptide aldehydes were used for this purpose. More recently, it became possible to synthesize compounds with increased potency and selectivity (Adams et al. 1998 Elofsson et al. 1999). Furthermore, based on the crystal structure of the yeast and bovine liver CP (Groll et al. 1997 Unno et al. 2002), molecular modeling can now be used to engineer improved inhibitors. 

As anticipated from their sequence similarity, the (non-catalytic) a- and the (catalytic) P-type subunits have the same fold (Lowe et al. 1995 Groll et al. 1997) a four-layer a+p structure with two antiparallel five-stranded P sheets, flanked on one side by two, on the other side by three a helices. In the P-type subunits, the P-sheet sandwich is closed at one end by four hairpin loops and open at the opposite end to form the active-site cleft the cleft is oriented towards the inner surface of the central cavity. In the a-type subunits an additional helix formed by an N-terminal extension crosses the top of the P-sheet sandwich and fills this cleft. Initially, the proteasome fold was believed to be unique however it turned out to be prototypical of a new superfamily of proteins referred to as Ntn (N-terminal nucleophile) hydrolases (Brannigan et al. 1995). 

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