Proteasomal particle

Fig.6 Binding of the 1 IS and 19S regulators to the 20S core proteasome forms new proteasomal particles...

Components of the 19S proteasomal particle recognize polyUb chains on target proteins (Fig. 6-7). Deu-biqitinating enzymes then remove the Uh, which is recycled, and the target protein is dehvered to the catalytic core (in an ATP-dependent fashion) where it is degraded. 

Three other components that my laboratory has identified and partially purified from Fraction 2 of reticulocytes, termed CF1-CF3, are involved in the degradation of proteins ligated to ubiquitin [24]. These are apparently subcomplexes of the 26S proteasome, a large ATP-dependent protease complex first described by Re-chsteiner and co-workers [25], CF3 is identical to the 20S proteasome core particle [26], while CFl and CF2 may be similar to the base and lid subcomplexes of the 19S regulatory particle of the 26S proteasome, described more recently by the Finley laboratory [27], In hindsight, the reason for finding subcomplexes, rather than the complete 26S complex in Fraction 2 was technical we have routinely prepared Fraction 2 from ATP-depleted reticulocytes [20], under which conditions the 26S proteasome dissociates to its subcomplexes. We found that incubation of the three subcomplexes in the presence of ATP promotes their assembly to the 26S proteasome [24, 26]. The role of ATP in the assembly of the 26S proteasome complex remains unknown. 

Glickman, M. H. et al. A subcomplex of the proteasome regulatory particle required for ubiquitin-conjugate degradation and related to the COP9-... 

Rubin, D. M., Glickman, M. H., Larsen, C. N., Dhruvakumar, S., and Finley, D. Active site mutants in the six regulatory particle ATPases reveal multiple roles for ATP in the proteasome. EMBO J. 1998, 17, 4909-4919. 

Braun, B. C. et al. The base of the proteasome regulatory particle exhibits chaperone-like activity. Nat Cell Biol 1999, 3, 221-6. 

Arendt, C. S. and Hoghstrasser, M. Eukaryotic 20S proteasome catalytic subunit propeptides prevent active site inactivation by N-terminal acetylation and promote particle assembly. Embo J 1999, 18, 3575-85. 

Kimuea, Y. et al. N-Terminal modifications of the 19S regulatory particle subunits of the yeast proteasome. Arch Biochcm Biophys 2003, 409, 341-8. 

ATP-dependent proteases are complex proteolytic machines. They are present in eubacteria, archaebacteria, in eukaryotic organelles and, as the 20S or 26S proteasome, in the eukaryotic cytosol and nucleoplasm. The activators of all known ATP-dependent proteases are related. They all contain an AAA(+) ATPase domain as a module (Neuwald et al. 1999) and are thought to assemble into hexameric particles or, in the case of 26S proteasomes, are present in six variants in the 19S activators (Glickman et al. 1999). Like the ATPases, the proteolytic components of the ATP-dependent proteases form higher order complexes, but unlike for the ATPases, the symmetry of the protease assemblies varies, and the folds of the subunits need not be related. ClpP is a serine protease, FtsH a metalloprotease, and HslV and the proteasomes from archaebacteria and eubacteria are threomne proteases. 

All 12 active sites of HslV are located on the inner walls of the hollow particle. In the E. coli particle, each active site has neighboring active sites 28 A away on the same ring and 22 A and 26 A away on the opposite ring. The environment of the nucleophilic Thrl looks similar to that in proteasomes, and the presence of a (putatively protonated) lysine residue near the active site probably helps to lower the pKa of the N-terminal a-amino group so that it is present in the unprotonated form, which can act as the general base to accept a proton from Thrl. 

Jager, S., Groll, M., Huber, R., Wolf, D. H., and Heinemeyer, W. Proteasome beta-type subunits unequal roles of propeptides in core particle maturation and a hierarchy of active site function./. Mol. Biol. 1999, 291, 997-1013. 

Kohler, A., Cascio, P., Leggett, D. S., Woo, K. M., Goldberg, A. L., and Finley, D. The axial channel of the proteasome core particle is gated by the Rpt2 ATPase and controls both substrate entry and product release. Molecular Cell 2001, 7, 1143-1152. 

I 7 7 Proteasome Regulator, PA700 (19S Regulatory Particle) Tab. n.1. Component Subunits of PA700/19S RP. 

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