Identification of novel ubiquitin receptors on the 26S proteasome by photo-crosslinking mass spectrometry.

The 26S proteasome is the endpoint of the ubiquitin-proteasome system, an essential pathway for maintaining cellular homeostasis through targeted degradation of misfolded, damaged, and obsolete proteins. Substrates labeled with ubiquitin are directed to the 26S proteasome by binding to one or more ubiquitin receptors. However, ubiquitin-dependent degradation occurs even when the canonical receptor sites are mutated, suggesting the presence of additional, unidentified binding sites. Here we created photo-crosslinkable probes for ubiquitin interactions by incorporating the unnatural amino acid p-benzoyl-L-phenylalanine into ubiquitin. We show that these probes can be used to measure apparent affinities for known receptors and to reveal novel ubiquitin-binding sites on the yeast 26S proteasome. Through photo-crosslinking mass-spectrometry experiments we identified a groove on the top of the proteasome, formed by Rpn2, Rpn9, Rpn10, and Rpn12, that serves as an additional ubiquitin-binding interface. Our photo-crosslinkable probes thus serve as versatile tools for the characterization of ubiquitin-protein interactions and the identification of ubiquitin-binding domains.