Optimized proteomic analysis on gels of cell-cell adhering junctional membrane proteins

A high level of structural organization of functional membrane domains in very narrow regions of a plasma membrane is crucial for the functions of plasma membranes and various other cellular functions. Conventional proteomic analyses are based on total soluble cellular proteins. Thus, because of insolubility problems, they have major drawbacks for use in analyses of low-abundance proteins enriched in very limited and specific areas of cells, as well as in analyses of the membrane proteins in two-dimensional gels. We optimized proteomic analyses of cell-cell adhering junctional membrane proteins on gels. First, we increased the purity of cell-cell junctions, which are very limited and specific areas for cell-cell adhesion, from hepatic bile canaliculi. We then enriched junctional membrane proteins via a guanidine treatment; these became selectively detectable on two- dimensionally electrophoresed gels after treatment with an extremely high concentration of NP-40. The framework of major junctional integral membrane proteins was shown on gels. These included six novel junctional membrane proteins of type I, type II, and tetraspanin, which were identified by mass spectrometry and by a database sequence homology search, as well as 12 previously identified junctional membrane proteins, such as cadherins and claudins.