Limited proteolysis combined with isotope labeling and quantitative LC-MALDI MS for monitoring protein conformational changes: a study on calcium-binding sites of cardiac Troponin C Original Research Article

Studies of protein–protein and protein–ligand interactions are important for understanding biological functions of proteins. A new technique based on the partial proteolysis of proteins combined with quantitative mass spectrometry is developed as a means of tracking structural changes after the formation of a protein–ligand complex. In this technique, a protein of interest with and without the binding of a ligand is digested with an enzyme to generate a set of peptides, followed by separation of the peptides by liquid chromatography. Matrix-assisted laser desorption ionization (MALDI) mass spectrometry (MS) is used to identify chromatographically separated peptides, and locate their sequence alignments in the parent protein. Using an isotopically labeled protein as a sample against an unlabeled protein standard, quantitative information can be gathered. This overcomes the inherent lack of quantitative capability of MALDI MS. The utility of the technique to investigate protein–ligand interactions is demonstrated in a model system involving calcium binding to cardiac Troponin C (cTnC). Using this technique, the general location of the three calcium-binding sites of cTnC can be determined by using several different enzymes to generate overlapping peptide maps of cTnC.