Diagram showing three transmembrane proteins involved in the formation of zonula occludens: occludin, claudin, and junctional adhesion molecule (JAM). Occludin and claudin have four transmembrane domains with two extracellular loops, but JAM has only a single transmembrane domain, and its extracellular portion possesses two immunoglobulinlike loops. Several major associated proteins of the occluding junction and their interactions with each other are visible. Note that one of the associated proteins, ZO-1, interacts with the cell cytoskeleton binding actin filaments.
Several proteins are involved in the formation of zonula occludens strands.
Zonula occludens strands correspond to the location of the rows of transmembrane proteins. Three major groups of transmembrane proteins are found in the zonula occludens (Fig. 5.16; Table 5.3):  Occludin, a 60-kilodalton protein, was the first protein identified in the zonula occludens. It participates in maintaining the barrier between adjacent cells as well as the barrier between the apical and lateral domains. Occludin is present in most occluding junctions. However, several types of epithelial cells do not have occludin within their strands, but they still possess well-developed and fully functional zonulae occludentes.
 Claudins constitute a family of proteins (20 to 27 kilodaltons) that have recently been identified as integral components of zonula occludens strands. Claudins form the backbone of each strand. In addition, claudins (especially claudin-2 and claudin-16) are able to form extracellular aqueous channels for the paracellular passage of ions and other small molecules. About 24 different members of the claudin family have been characterized to date. Mutations in the gene encoding claudin-14 have been recently linked to human hereditary deafness. A mutated form of claudin-14 causes an increased permeability of zonula occludens in the organ of Corti (receptor of hearing), affecting generation of action potentals.
 Junctional adhesion molecule (JAM) is a 40kilodalton protein that belongs to the immunoglobulin superfamily (IgSF). JAM does not itself form a zonula occludens strand but is instead associated with claudins. It is involved in the formation of occluding junctions in endothelial cells as well as between endothelial cells and monocytes migrating from the vascular space to the connective tissue.
The extracellular portions of these transmembrane proteins function as a zipper and seal the intercellular space between two adjacent cells, thus creating a barrier against paracellular diffusion. The cytoplasmic portions of all three proteins contain a unique amino acid sequence that attracts regulatory and signaling proteins called PDZ-domain proteins. These proteins include the zonula occludens proteins ZO-1, ZO-2, and ZO-3 (see Fig. 5.16). Occludin and claudins interact with the actin cytoskeleton through ZO-1 and ZO-3. Regulatory functions during the formation of the zonula occludens have been suggested for all ZO proteins. In addition, ZO-1 is a tumor suppressor, and ZO-2 is required in the epidermal growth factor–receptor signaling mechanism. The ZO-3 protein interacts with ZO-1 and the cytoplasmic domain of occludins. The proteins localized in the region of the zonula occludens are summarized in Table 5.3. Many pathogenic agents, such as cytomegalovirus and cholera toxins, act on ZO-1 and ZO-2, causing the junction to become permeable.
The zonula occludens separates the luminal space from the intercellular space and connective tissue  compartment.