Abstract
By raising monoclonal antibodies to the apical surface of Caco-2 cells we have identified a membrane protein (p100) that internalizes and recycles constitutively between the apical plasma membrane and endosomes in the apical cytoplasm. By applying tracers bound to the transferrin receptor, which internalizes and recycles back to the basolateral border, we demonstrate that the apical endosomes containing p100 include a subset of multivesticular bodies (MVB), which are also accessible to proteins arriving from the basolateral endosome. Tracers bound to EGF receptors and alpha-2-macroglobulin, which internalize from the basolateral border and are degraded, probably in lysosomes, also pass through the p100-containing MVB. These studies therefore suggest that the apical cytoplasm of Caco-2 cells contains a population of MVB capable of receiving membrane proteins trafficking in from both apical and basolateral borders and then routing them to a variety of cell surface and intracellular destinations. The differential distribution of apical and basolateral tracers within the 50-nm-diameter tubules connected to these p100-positive apical MVB suggests that the destination of proteins trafficking from the MVB back to apical and basolateral surfaces is determined by the tubules to which they gain access.