Immunoprecipitation (IP) represents a widely utilized biochemical method to isolate a specific protein from a complex mixture taking advantage of an antibody that specifically recognizes that particular target molecule. This procedure is extremely versatile and can be applied to concentrate a specific protein, to identify interacting partners in complex with it or to detect post-translational modifications. The mitotic protein monoclonal 2 (MPM-2) is an antibody originally raised against extracts of synchronized mitotic HeLa cells to identify proteins selectively present in mitotic, and not in interphase-cells (Davis et al., 1983). MPM-2 recognizes phosphorylated serine or threonine residues followed by proline (pS/T-P), consensus epitopes generated by the concerted action of proline-directed protein kinases and phosphatases (Lu et al., 2002). These reversible phosphorylation events have emerged to control various cellular processes by promoting conformational changes on the target that are not simply due to the phosphorylation event per se. These motifs, once phosphorylated, are able to recruit Pin1 (Peptidyl-prolyl Isomerase NIMA interacting protein 1) (Lu et al., 1996; Lu and Zhou, 2007), a chaperone which drives the cis/trans isomerization reaction on the peptide bond, switching the substrate between functionally diverse conformations (Lu, 2004; Wulf et al., 2005). This protocol describes a general MPM-2 based immunoprecipitation strategy using the scaffolding molecule postsynaptic density protein-95 (PSD-95) (Chen et al., 2005), a neuronal Pin1 target (Antonelli et al., 2016), as an example to illustrate the detailed procedure.
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