Proteomic analysis of the membrane palmitoylated protein-4 (MPP4)-associated protein complex in the retina

Membrane palmitoylated protein-4 (MPP4) is a retina-specific scaffolding protein of the membrane-associated guanylate kinase family that has been implicated in organizing presynaptic protein complexes in the photoreceptor ribbon synapse. To isolate the components of this complex we applied a proteomic approach based on immunoaffinity chromatography with a monoclonal anti-MPP4 antibody followed by two-dimensional electrophoresis and mass spectrometry. Among the identified molecules were previously reported proteins of the MPP4 scaffolding complex including adaptor proteins Veli3 and Psd95. Here we demonstrate a selective association between MPP4 and the Psd95-beta isoform that is mediated by interaction of their N-terminal L27 domains. In addition, we have identified recoverin and Hsc70 as novel associated proteins of the MPP4 multiprotein complex in the retina. This study demonstrates the utility of anti-MPP4 antibody precipitation for the elucidation of the MPP4-associated protein complex, which is essential in understanding its precise role in signal transmission at the photoreceptor synapse.