Cingulin, a new peripheral component of tight junctions

Abstract

The tight junction (zonula occludens), a belt-like region of contact between cells of polarized epithelia, serves as a selective barrier to small molecules and as a total barrier to large molecules^1,2, and is involved in the separation between lumenal and basolateral compartments of the epithelium^3,4. In the electron microscope, tight junctions show focal regions of apparent fusion between the adjoining cell membranes, and freeze-fractured membranes display an elaborate network of branching and anastomosing strands^1,5–8. Very little is known about the molecular composition and architecture of tight junctions. The first specific zonula occludens-associated protein, designated ZO-1, has recently been identified in mammalian epithelial and endothelial cells⁹. Here we describe the identification and purification of a new component of this junc-tional complex in avian brush-border cells, which we name cingulin. Cingulin is an acidic, heat-stable protein, with a highly elongated shape. Immunofluorescence and immunoelectron microscopy of brush-border cells with anti-cingulin antibodies show that cingulin is localized in the apical zone of the terminal web, at the endofacial surfaces of the zonula occludens.