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The tight junction is a highly crosslinked two-dimensional macromolecular lattice of membrane proteins coupled to an underlying F-actin network by associated cytoplasmic proteins. It is located at the apicolateral region of the plasma membrane of epithelial cells. The claudins are a family of tight junction membrane proteins. The claudins oligomerize to form a double-stranded anastomosing network that is the structural basis for the tight junction permeability barrier (see Claudin Network figure). Claudin proteins on apposing epithelial cells bind together to form transcellullar dimers. The dimers of some members of the claudin family seal the tight junction, creating a barrier to paracellular solute movement (barrier-forming claudins). Other members of the claudin family form dimers that create small pores (5-6 Angstrom radius) in the tight junction barrier (pore-forming claudins). These claudin pores are charge- and size-selective (see Leak and Pore Pathways ). Other proteins are also contained within the tight junction structure including Tight junction-Associated Marvel Proteins (TAMPs - occludin, tricellulin, and Marvel D3) and Junctional Adhesion Molecule-A (see Tight Junction Structure figure). These proteins are crosslinked by tight junction-associated cytoplasmic proteins such as those in the ZO protein family, primarily ZO-1 and ZO-2. These cytoplasmic tight junction proteins have binding sites for multiple tight junction membrane proteins. The ZO proteins also bind to F-actin filaments, thereby linking the tight junction to the underlying F-actin network.
Looking down on the apical surface of MDCK cells, the tight junction proteins are localized along the plasma membrane at the apicolateral border in what is described as a "chicken wire" pattern (see Immunolocalization of Claudin Protein figure). The bright green color indicates the location of claudin 1 protein. This appearance is typical of tight junction proteins.
Here is a link to a video (https://drive.google.com/file/d/17l-qHGOuhm8MK7a3Tt79qVSenXBYAzv0/view?usp=sharing) showing the organization and localization of the ZO-1 protein and F-actin as you focus down through a population of MDCK cells from the apical membrane down to and through the basolateral membrane. The ZO-1 protein is labeled green and the F-actin is labeled red. The tight junction structure (yellow color) is located at the apicolateral region of each cell. The yellow color indicates that ZO-1 protein and F-actin are colocalized in the tight junction. Once past the tight junction, as you focus down through the cell you see primarily the red color indicating the presence of F-actin but little ZO-1 protein. In the bottom part of the cell (basal), the red color exhibits a striated pattern which represents F-actin stress fibers that contribute to the attachment of MDCK cells to the underlying extracellular matrix.
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