Apical membrane aminopeptidase appears at site of cell-cell contact in cultured kidney epithelial cells.

A dog kidney epithelial cell line (MDCK), grown in monolayer, displayed in vitro an asymmetric localization of surface proteins. Aminopeptidase [alpha-aminoacylpeptide hydrolase (microsomal), EC 3.4.11.2] was found only in the apical face whereas Na+, K+-ATPase (ATP phosphohydrolase, EC 3.6.1.3) was found in the basolateral faces. These two faces are delineated by the junctional complex at which close cell-cell contact occurs. alpha-Actinin, a protein associated with plasma membranes, was concentrated near the region of cell-cell contact. When membrane proteins in the apical surface were crosslinked and subsequently removed from the surface by endocytosis, crosslinked antigens reappeared in the apical face at the region of cell-cell contact. Antigens that were not crosslinked were also (re)inserted in the same region. This process was not affected by cycloheximide, presumably because a large pool of apical membrane proteins (observed in small cytoplasmic vesicles) was used to replace the endocytosed antigens. It is psoposed that the region containing the junctional complex is involved in guiding apical membrane proteins to their final location.