Metabolic labeling of antigens can be achieved by supplementation of the cell culture medium with radioactive amino acid precursors such as [35S]methionine during the incubation period of target cells. In this protocol, intracellular unlabeled methionine levels are reduced by starvation of cells for 0.5-1 h before the addition of labeled [35S]methionine and incubation for 0.5-4 h. Upon completion of the metabolic labeling process, cells can be prepared for immunoprecipitation by lysis or alternatively pelleted and frozen for cell lysate preparations at a later time.
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