Straightforward and de Novo Peptide Sequencing by MALDI-MS/MS Using a Lys-N Metalloendopeptidase

In this work, we explore the potential of the metalloendopeptidase Lys-N for MALDI-MS/MS proteomics applications. Initially we digested a HEK293 cellular lysate with Lys-N and, for comparison, in parallel with the protease Lys-C. The resulting peptides were separated by strong cation exchange to enrich and isolate peptides containing a single N-terminal lysine. MALDI-MS/MS analysis of these peptides yielded CID spectra with clear and often complete sequence ladders of b-ions. To test the applicability for de novo sequencing we next separated an ostrich muscle tissue protein lysate by one-dimensional SDS-PAGE. A protein band at 42 kDa was in-gel digested with Lys-N. Relatively straightforward sequencing resulted in the de novo identification of the two ostrich proteins creatine kinase and actin. We therefore conclude that this method that combines Lys-N, strong cation exchange enrichment, and MALDI-MS/MS analysis provides a valuable alternative proteomics strategy.