Ac-LysargiNase Complements Trypsin for the Identification of Ubiquitinated Sites

Mass spectrometry (MS)-based identification of ubiquitinated sites requires trypsin digestion prior to MS analysis, and a signature peptide was produced with a diglycine residue attached to the ubiquitinated lysine (K-ε-GG peptide). However, the missed cleavage of modified lysines by trypsin results in modified peptides with increased length and charge, whose detection by MS analysis is suppressed by the vast majority of internally unmodified peptides. LysargiNase, the mirrored trypsin, is reported to cleave before lysine and arginine residues and to be favorable for the identification of methylation and phosphorylation, but its digestive characteristics related to ubiquitination are unclear. Herein, we tested the capacity of the in-house developed acetylated LysargiNase (Ac-LysargiNase) with high activity and stability, for cleaving ubiquitinated sites in both the seven types of ubiquitin chains and their corresponding K-ε-GG peptides. Interestingly, Ac-LysargiNase could efficiently cleave the K63-linked chain but had little effect on the other types of chains. Additionally, Ac-LysargiNase had higher exopeptidase activity than trypsin. Utilizing these features of the paired mirror proteases, a workflow of trypsin and Ac-LysargiNase tandem digestion was developed for the identification of ubiquitinated proteins. Through this method, the charge states and ionization capacity of the unmodified peptides were efficiently reduced, and the identification of modified sites was consequently increased by 30% to 50%. Strikingly, approximately 15% of the modified sites were cleaved by Ac-LysargiNase, resulting in shorter K-ε-GG peptides for better identification. The enzyme Ac-LysargiNase is expected to serve as an option for increasing the efficiency of modified site identification in ubiquitome research.