Simple, scalable, and ultrasensitive tip-based identification of protease substrates.

Proteases are in the center of many diseases, and consequently, proteases and their substrates are important drug targets as represented by an estimated 5-10% of all drugs under development. Mass spectrometry has been an indispensable tool for the discovery of novel protease substrates, particularly through the proteome-scale enrichment of so-called N-terminal peptides representing endogenous protein N termini. Methods such as combined fractional diagonal chromatography (COFRADIC) and, later, terminal amine isotopic labeling of substrates (TAILS) have revealed numerous insights into protease substrates and consensus motifs. We present an alternative and simple protocol for N-terminal peptide enrichment, based on charge-based fractional diagonal chromatography (ChaFRADIC) and requiring only well-established protein chemistry and a pipette tip. Using iTRAQ-8-plex, we quantified on average 2,073 ± 52 unique N-terminal peptides from only 4.3 μg per sample/channel, allowing the identification of proteolytic targets and consensus motifs. This high sensitivity may even allow working with clinical samples such as needle biopsies in the future. We applied our method to study the dynamics of staurosporine-induced apoptosis. Our data demonstrate an orchestrated regulation of specific pathways after 1.5 h, 3 h, and 6 h of treatment, with many important players of homeostasis targeted already after 1.5 h. We additionally observed an early multilevel modulation of the splicing machinery both by proteolysis and phosphorylation. This may reflect the known role of alternative splicing variants for a variety of apoptotic genes, which seems to be a driving force of staurosporine-induced apoptosis.