Technologies for plasma membrane proteomics

Cell-cell and intracellular signaling are critical mechanisms by which an organism can respond quickly and appropriately to internal or environmental stimuli. Transmission of the stimulus to effector proteins must be coordinated, rapid and transient such that the response is not exaggerated and the overall balance of the cell or tissue is retained. Proteomics technology has traditionally been adept at analyzing effector proteins (such as cytoskeletal and heat shock proteins, and those involved in metabolic processes) in studies examining the effects of altered environmental or nutritional conditions, drugs, or genetic manipulation, since these proteins are often highly abundant, soluble and therefore amenable to analysis. Conversely, the proteins mediating the transmission of the signal have been generally under-represented, typically because of their low abundance. One mechanism that has overcome this to some extent is the advent of very high-resolution phosphoproteomics techniques, which have enabled temporal profiling of intracellular signal pathways via quantitative assessment of peptide phosphorylation sites. One group of proteins, however, that still remains under-represented in proteomics studies are those found in the plasma membrane (PM). Such proteins are crucial in sensing changes in the external environment and in stimulating the transmission of the signal intracellularly. This review examines PM proteins and appraises the proteomics approaches currently available for providing a comprehensive analysis of these crucial mediators of signal pathways. We discuss different strategies for enrichment and solubilization of these proteins and include discussion on cross-linking of PM complexes and glycoproteomics as the basis for purification prior to proteomic analyses.