Protein Termini and Their Modifications Revealed by Positional Proteomics

A myriad of co- and post-translational modifications occur at protein N- and C-termini, resulting in an extra layer of proteome complexity and an additional source of protein regulation. Here, we review N- and C-terminal modifications and the contemporary positional proteomics techniques used to isolate protein terminal peptides from complex protein mixtures and characterize their diversity and occurrence in biological systems. Furthermore, these degradomics strategies--often referred to as N- and C-terminomics--represent dedicated high-throughput techniques to study proteolysis in dynamic living systems. Over the past decade, terminomics studies have provided indispensable information on the functional states of individual proteins, cell types, tissues, and biological processes and delivered fundamental new data for the Human Proteome Project, including high confidence identifications of many so-called "missing proteins", which had not been identified by traditional proteomics analyses.