Ubiquitin binding proteins protect ubiquitin conjugates from disassembly.

AbstractAs a step in their turnover proteins in eukaryotic cells are coupled to a small protein, ubiquitin, before they are recognised by 26S proteasomes and degraded. However, cells also contain many deubiquitinating enzymes, which can rescue proteins by cleaving off the ubiquitin chains. Here we report that three ubiquitin binding proteins, Rhp23, Dph1 and Pus1, from fission yeast can protect multiubiquitin conjugates against deubiquitination. This protection depends on the ubiquitin binding domains and may promote degradation of ubiquitinated proteins

Subunit arrangement in the human 20S proteasome

In human 20S proteasomes two copies of each of seven different α-type and seven different β-type subunits are assembled to form a stack of four seven-membered rings, giving the general structure α(1–7), β(1–7), β(1–7), α(1–7). By means of immunoelectron microscopy and chemical crosslinking of neighboring subunits, we have determined the positions of the individual subunits in the proteasome. The topography shows that for the trypsin-like, the chymotrypsin-like, and the postglutamyl cleaving activities, the pairs of β type subunits, which are thought to form active sites, are nearest neighbors

Mammalian 26S proteasomes remain intact during protein degradation

SummaryIt has been suggested that degradation of polyubiquitylated proteins is coupled to dissociation of 26S proteasomes. In contrast, using several independent types of experiments, we find that mammalian proteasomes can degrade polyubiquitylated proteins without disassembling. Thus, immobilized, 35S-labeled 26S proteasomes degraded polyubiquitylated Sic1 and c-IAP1 without releasing any subunits. In addition, it is predicted that if 26S proteasomes dissociate into 20S proteasomes and regulatory complexes during a degradation cycle, the reassembly rate would be limiting at low proteasome concentrations. However, the rate with which each proteasome degraded polyubiquitylated Sic1 was independent of the proteasome concentration. Likewise, substrate-dependent dissociation of 26S proteasomes could not be detected by nondenaturing electrophoresis. Lastly, epoxomicin-inhibited 20S proteasomes can trap released regulatory complexes, forming inactive 26S proteasomes, but addition of epoxomicin-inhibited 20S proteasomes had no effect on the degradation of either polyubiquitylated Sic1 or UbcH10 by 26S proteasomes or of endogenous substrates in cell extracts