SILAC

SILAC (Stable isotope labelling with amino acids in cell culture) is a mass spectrometry-based technique developed in the group of Matthias Mann to detect differences in protein abundance between two samples (Ong et al., 2002). Two populations of cells are cultivated in cell culture. One of the cell populations is fed with growth medium containing normal amino acids. In contrast, the growth medium of the second cell population contains amino acids labeled with stable (non-radioactive) heavy isotopes. For example, the medium can contain arginine labeled with six carbon-13 atoms (13C) instead of the normal carbon-12 (12C). When the cells are growing in this medium, they incorporate the heavy arginine into all of their proteins. Therefore, all of the arginine containing peptides are now 6 Da heavier than their normal counterparts. The trick is that the proteins from both cell populations can be combined and analyzed together by mass spectrometry. Pairs of chemically identical peptides of different stable-isotope composition can be differentiated in a mass spectrometer owing to their mass difference. The ratio of peak intensities in the mass spectrum for such peptide pairs accurately reflects the abundance ratio for the two proteins. SILAC has emerged as a very powerful method to study cell signaling and protein-protein interaction.