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The proteasome controls the concentrations of most proteins in eukaryotic cells. It recognizes its protein substrates through ubiquitin tags and initiates degradation at disordered regions within the substrate. Here we show that the proteasome has pronounced preferences for the amino acid sequence of the regions at which it initiates degradation. Specifically, proteins in which the initiation regions have biased amino acid compositions show longer half-lives in yeast than proteins with unbiased sequences in the regions. The relationship is also observed on a genomic scale in mouse cells. These preferences affect the degradation rates of proteins in vitro, can explain the unexpected stability of natural proteins in yeast and may affect the accumulation of toxic proteins in disease. We propose that the proteasome's sequence preferences provide a second component to the degradation code and may fine-tune protein half-life in cells.

The eukaryotic 26S proteasome controls cellular processes by degrading specific regulatory proteins. Most proteins are targeted for degradation by a signal or degron that consists of two parts: a proteasome-binding tag, typically covalently attached polyubiquitin chains, and an unstructured region that serves as the initiation region for proteasomal proteolysis. Here we have characterized how the arrangement of the two degron parts in a protein affects degradation. We found that a substrate is degraded efficiently only when its initiation region is of a certain minimal length and is appropriately separated in space from the proteasome-binding tag. Regions that are located too close or too far from the proteasome-binding tag cannot access the proteasome and induce degradation. These spacing requirements are different for a polyubiquitin chain and a ubiquitin-like domain. Thus, the arrangement and location of the proteasome initiation region affect a protein's fate and are important in selecting proteins for proteasome-mediated degradation.

The proteasome controls the turnover of many cellular proteins. Two structural features are typically required for proteins to be degraded: covalently attached ubiquitin polypeptides that allow binding to the proteasome and an unstructured region in the targeted protein that initiates proteolysis. Here, we have tested the degradation of model proteins to further explore how the proteasome selects its substrates. Using purified yeast proteasome and mammalian proteasome in cell lysate, we have demonstrated that the two structural features can act in trans when separated onto different proteins in a multisubunit complex. In such complexes, the location of the unstructured initiation site and its chemical properties determine which subunit is degraded. Thus, our findings reveal the molecular basis of subunit specificity in the degradation of protein complexes. In addition, our data provide a plausible explanation for how adaptor proteins can bind to otherwise stable proteins and target them for degradation.