A functional assay has been developed to identify cell surface proteins involved in the formation of epithelial tight junctions. Transepithelial electrical resistance was used to measure the presence of intact tight junctions in monolayers of Madin-Darby canine kidney (MDCK) cells cultured on nitrocellulose filters. The strain I MDCK cells used have a transmonolayer resistance greater than 2,000 ohm . cm2. When the monolayers were incubated at 37 degrees C without Ca2+, the intercellular junctions opened and the transmonolayer resistance dropped to the value of a bare filter, i.e., less than 40 ohm . cm2. When Ca2+ was restored, the cell junctions resealed and the resistance recovered rapidly. Polyclonal antibodies raised against intact MDCK cells inhibited the Ca2+-dependent recovery of electrical resistance when applied to monolayers that had been opened by Ca2+ removal. Cross-linking of cell surface molecules was not required because monovalent Fab' fragments also inhibited. In contrast, a variety of other antibodies that recognize specific proteins on the MDCK cell surface failed to inhibit the recovery of resistance. Monoclonal antibodies have been raised and screened for their ability to inhibit resistance recovery. One such monoclonal antibody has been obtained that stained the lateral surface of MDCK cells. This antibody, rr1, recognized a 118-kD polypeptide in MDCK cell extracts and an 81-kD fragment released from the cell surface by trypsinization in the presence of Ca2+. Sequential immunoprecipitation with antibody rr1 and a monoclonal antibody to uvomorulin showed that this polypeptide is related to uvomorulin. The role of uvomorulin-like and liver cell adhesion molecule (L-CAM)-like polypeptides in the establishment of the epithelial occluding barrier is discussed.

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