Resin-assisted enrichment of N-terminal peptides for characterizing proteolytic processing.

A resin-assisted enrichment method has been developed for specific isolation of protein N-terminal peptides to facilitate LC-MS/MS characterization of proteolytic processing, a major form of posttranslational modifications. In this method, protein thiols are blocked by reduction and alkylation, and protein lysine residues are converted to homoarginines. Protein N-termini are selectively converted to reactive thiol groups, and the thiol-containing N-terminal peptides are then captured by a thiol-affinity resin with high specificity (>97%). The efficiencies of these sequential reactions were demonstrated to be nearly quantitative. The resin-assisted N-terminal peptide enrichment approach was initially applied to a cell lysate of the filamentous fungus Aspergillus niger. Subsequent C-MS/MS analyses resulted in the identification of 1672 unique protein N-termini or proteolytic cleavage sites from 690 unique proteins.