Immunoprecipitation of Radiolabeled Cells Protocols

Most powerful and convincing method to determine if a specific protein is phosphorylated in a physiologically relevant manner is to assay phosphorylation in situ. The procedure described involves incubating cultured cells (e.g., primary neuronal cultures or transfected cells) or tissue preparations (e.g., hippocampal slices) with [32P]orthophosphate, which is then taken up by the cells or tissues and incorporated into the γ-phosphate position of ATP. - [Read Detection of Protein Phosphorylation in Tissues and Cells Protocol]

Coimmunoprecipitation is most commonly used to test whether two proteins of interest are associated in vivo, but it can also be used to identify novel interacting partners of a target protein. In both cases, the cells, which may have been labeled with [35S]methionine, are harvested and lysed under conditions that preserve protein-protein interactions. The target protein is specifically immunoprecipitated from the cell extracts, and the immunoprecipitates are fractionated by SDS-PAGE. - [Read Identification of Associated Proteins by Coimmunoprecipitation Protocol]

Cells incorporate 35S-methionine or cysteine during the protein synthesis. Thus it is essential to use Met,Cys-free medium and dialyzed FCS during the labeling. Short period of incubation with 35S-methionine or cysteine will result in radiolabeling (pulse), and additional incubation with excess concentration of unlabeled Met+Cys (chase) is needed for complex glycoproteins like integrins to get expressed as a maturated form. - [Read Metabolic Labeling & Immunoprecipitation Protocols]