No abstract

Ubiquitin-specific proteases (USPs) are papain-like isopeptidases with variable inter- and intramolecular regulatory domains. To understand the effect of these domains on USP activity, we have analyzed the enzyme kinetics of 12 USPs in the presence and absence of modulators using synthetic reagents. This revealed variations of several orders of magnitude in both the catalytic turnover (k(cat)) and ubiquitin (Ub) binding (K(M)) between USPs. Further activity modulation by intramolecular domains affects both the k(cat) and K(M), whereas the intermolecular activators UAF1 and GMPS mainly increase the k(cat). Also, we provide the first comprehensive analysis comparing Ub chain preference. USPs can hydrolyze all linkages and show modest Ub-chain preferences, although some show a lack of activity toward linear di-Ub. This comprehensive kinetic analysis highlights the variability within the USP family.

Thiolysine‐mediated chemical ligation has generated fluorescence polarisation assay reagents based on isopeptide‐linked ubiquitin‐like protein conjugates. These have been used to monitor the activity of ubiquitin(‐like) proteases. Thus, it is now possible to generate assay reagents that contain substrate‐derived elements around the isopeptide linkage, with no practical limitation.

SUMMARY Ubiquitin (Ub) signaling is a diverse group of processes controlled by covalent attachment of small protein Ub and the polyUb chains to a range of cellular protein targets. Best documented Ub signaling pathway is the one that delivers polyUb-proteins to the 26S proteasome for degradation. However, studies of molecular interactions involved in this process have been hampered by the transient and hydrophobic nature of these interactions and the lack of tools to study them. Here, we develop Ub-phototrap (UbPT), a synthetic Ub variant containing a photoactivatable crosslinking side chain. Enzymatic polymerization into chains of defined lengths and linkage types provided a set of reagents that led to identification of Rpn1 as a third proteasome ubiquitin-associating subunit that coordinates docking of substrate shuttles, unloading of substrates, and anchoring of polyUb-conjugates. Our work demonstrates the value of UbPT and we expect that its future uses will help define and investigate the ubiquitin interactome.