Let the Peptides Shine: SOX (Sulfonamido-OXine)-Labelled Peptides for Direct Kinase and Phosphatase Monitoring

Abstract

Protein kinases and their cognate phosphatases are central players in cellular signalling events. They initiate and relay phosphorylation signals along intracellular pathways using Mg 2+ and ATP. Defects in kinase/phosphatase signalling are implicated in many diseases, including cancer, neurodegenerative conditions and metabolic disorders. Rapid and economical methods for measuring kinase activities are critically needed as diagnostic tools and for the evaluation of kinase inhibitors. However, most of the commonly employed approaches are unsuitable for direct and quantitative measurement of kinase activities in biological settings. Chelation-Enhanced Fluorescence (CHEF) is a physicochemical "turn-on" mechanism in which self-sensing probes become brighter upon chelation of metal ions. CHEF sensors incorporating the fluorophore sulfonamido-oxine (SOX) can accurately and quantitatively report on kinase activity. The development of an array of SOX-peptides that undergo CHEF upon phosphorylation by their specific target kinase has allowed researchers to gain insights into biological processes including binding events, conformational constraints, enzyme activities, protein trafficking and localization. This review summarizes the current literature on peptides that probe kinase and phosphatase activity in realtime assays, operating through the CHEF effect. We focus on studies in which kinase activation is detected upon alcohol phosphorylation and Mg 2+ chelation to the SOX chromophore.