THE TIGHT JUNCTION PROTEIN ZO-1 ESTABLISHES A LINK BETWEEN THE TRANSMEMBRANE PROTEIN OCCLUDIN AND THE ACTIN CYTOSKELETON

The tight junction protein ZO-1 belongs to a family of multidomain pro teins known as the membrane-associated guanylate kinase homologs (MAGU Ks). ZO-1 has been demonstrated to interact with the transmembrane pro tein occludin, a second tight junction-specific MAGUK, ZO-2, and F-act in, although the nature and functional significance of these interacti ons is poorly understood. To further elucidate the role of ZO-1 within the epithelial tight junction, we have introduced epitope-tagged frag ments of ZO-1 into cultured MDCK cells and identified domains critical for the interaction with ZO-2, occludin, and F-actin. A combination o f in vitro and in vivo binding assays indicate that both ZO-2 and occl udin interact with specific domains within the N-terminal (MAGUK-like) half of ZO-1, whereas the unique proline-rich C-terminal half of ZO-1 cosediments with F-actin. Consistent with these observations, we foun d that a construct encoding the N-terminal half of ZO-1 is specificall y associated with tight junctions, whereas the unique C-terminal half of ZO-1 is distributed over the entire lateral surface of the plasma m embrane and other actin-rich structures. In addition, we have identifi ed a 244-amino acid domain within the N-terminal half of ZO-1, which i s required for the stable incorporation of ZO-1 into the junctional co mplex of polarized MDCK cells. These observations suggest that one fun ctional role of ZO-1 is to organize components of the tight junction a nd link them to the cortical actin cytoskeleton.