In-Depth Analysis of C Terminomes Based on LysC Digestion and Site-Selective Dimethylation.

Analysis of protein C termini is very important for functional annotations of proteomes, while proteome-wide C termini analysis still poses substantial challenges. Here we described a simple and robust strategy for specific isolation of protein C termini based on LysC digestion and site-selective dimethylation to deplete N-terminal and internal peptides by scavenger materials. The performance of LysC digestion and conditions of site-selective dimethylation and resin coupling were discussed in detail. Then the strategy was successfully applied to the characterization of protein C termini of HeLa cells. A total of 781 protein C termini were identified with a 300 μg digest in our study, among which 38.9% were actually not identifiable using current trypsin digestion-based methods due to their inappropriate peptide length for MS analysis, indicating that our method was highly complementary to the existing methods. The enrichment procedure was rapid and easy to operate and could afford a very good identification efficiency by obtaining the largest C termini data set of the human proteome with the least sample loading. This method was without bias toward physicochemical properties of peptides. Moreover, a peptide-centric database was first introduced to analyze protein C termini, which effectively improved the accuracy and speed of the database search. Therefore, our method can be used to effectively and selectively isolate protein C termini and contributes to the global annotation of C terminomes.