The PIs coated beads assay or “PIP-Beads” developed by Echelon Biosciences (Salt Lake City, USA) is a quick assay to recognize which PIs are able to bind to a given protein domain, in a quantitative way. It is much faster and cheaper than liposomes and more reproducible than PIP-strip assays. The “PIP-Beads” assay is a biochemical assay that basically involves an incubation of a purified protein or protein domain with the appropriate PI-coated set of beads. After washing, drying and resuspending the samples, they can be easily analyzed by SDS-PAGE separation. Phosphoinositides (PIs) have been characterized as important determinants of cell membrane domains, such as the apical and basolateral domains in epithelial polarized cells (Martin-Belmonte and Mostov, 2007), controlling membrane trafficking (Szentpetery et al., 2010) or determining the presynaptic or postsynaptic terminal in neurons, among other functions (Di Paolo and De Camilli, 2006). These phosphoinositides enriched membranes bring the proteomic machinery together, confers to these membrane their different identities and functions. This protein-PIs interaction in many cases involves direct binding of specific protein membrane domains with certain PIs. Some of these domains are characterized such as PH domains from phospholipase-C- or synaptotagmin-like C2 domains (Galvez-Santisteban et al., 2012), while some of them are not. To determine which PI is binding to a given protein domain, it is important to have a quick and efficient assay. The liposome binding assays are very good to establish the kinetic properties of binding, but they are expensive and permit only to test a few PIs per experiment. On the other hand, PIP-strip (phosphatidil-inositol-phosphate) based analysis is easy and fast, however the PIs are presented in a flat surface and the reproducibility is sometimes limited.
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