The role of uvomorulin in the formation of epithelial occluding junctions

We have used an assay for the recovery of transepithelial resistance to identify proteins involved in the formation of the epithelial occluding barrier. The occluding junctions of high resistance monolayers of strain I MDCK cells (greater than 2500 ohm X cm2) were opened briefly and reversibly by removal of Ca2+ from the bathing medium. We screened for monoclonal antibodies which could inhibit the recovery of resistance upon Ca2+ readdition. One such monoclonal antibody, rrl, was obtained which recognizes a uvomorulin-like (or L-CAM-like) polypeptide in MDCK cells. Uvomorulin may be primarily, if not entirely, responsible for the Ca2+ dependence of occluding junction integrity. The maintenance of transmonolayer resistance had the same divalent cation selectivity as the conformational change shown by the uvomorulin molecule. In contrast to its exclusive localization to the zonula adherens of small intestinal cells, we found uvomorulin to be distributed over much of the lateral plasma membrane of MDCK cells and dog hepatocytes. The role of extrajunctional uvomorulin in these cells is not understood. Treatment of intact, high resistance monolayers of MDCK cells with antibody rr1 failed to induce a loss of resistance even though the antibody had access to uvomorulin at the lateral surfaces. Thus, cell junctions must be partially disrupted to perturb the function of uvomorulin. We present a working model for the function of uvomorulin in the establishment of the epithelial occluding barrier.