Phosphoprotein B-50 and phosphoinositides in brain synaptic plasma membranes: a possible feedback relationship.

synthesized sucrase-isomaltase with the basolateral membrane fraction may have been due to a co-purification of basolateral membranes with transport vesicles for which no marker proteins are known. The differentiated colon carcinoma cell line Caco 2 represents a promising cell culture model for studying molecular mechanisms underlying intracellular membrane traffic and the biogenesis of surface membrane polarity in epithelial cells. The use of a cell culture system which closely mimicks the epithelial organization in vivo has a number of advantages in comparison with whole tissue as the experimental system. In particular, a cell culture system provides a homogeneous cell population and allows easy experimental manipulation. Although more difficult with cell cultures, subcellular fractionation of Caco 2 cells is feasible and can yield membrane fractions whose purity approaches that obtained with whole tissue. Progress in our understanding of the biosynthetic pathway of surface membrane proteins has mainly come from studies on virus maturation in Madin-Darby canine kidney cells (Simons & Fuller, 1985). I t will now be possible to investigate these mechanisms with endogenous membrane proteins in Caco 2 cells.