| ZO1 tight junction protein (TJP1) is a membrane-associated guanylate kinase (MAGUK) family scaffold that localizes to epithelial and endothelial tight junctions where it connects transmembrane junctional proteins with the cortical actin cytoskeleton and thereby contributes to barrier formation, cell polarity, and junctional signaling. ZO-1 contains three N‑terminal PDZ domains followed by an SH3 and guanylate kinase-like domain and a proline-rich C‑terminal region, an arrangement that enables simultaneous binding of multiple partners: claudins and junctional adhesion molecules engage the PDZ domains, occludin and ZO-2 bind within the MAGUK-like N‑terminal half, and F‑actin associates with the unique C‑terminal segment, creating a structural bridge between tight junction strands and the perijunctional actomyosin ring. This modular organization supports tight junction assembly by stabilizing claudin-based pores and occludin-containing strands at the apical lateral membrane while coupling them to the cytoskeleton, and a defined 244‑amino acid region within the N‑terminal half is required for stable incorporation of ZO-1 into mature junctional complexes. ZO-1 and its paralog ZO-2 are indispensable for early junction biogenesis and barrier competence, as loss of both proteins prevents formation of morphologically and functionally intact tight junctions and leads to embryonic lethality with defects in epithelial organization, yolk sac angiogenesis, and proliferation, emphasizing their central role in paracellular permeability control and tissue morphogenesis. At established junctions, ZO-1 stabilizes the solute barrier by maintaining the linkage between tight junction elements and the perijunctional actomyosin belt, selectively limiting flux of larger solutes while allowing claudin pore–mediated passage of small ions, and its depletion causes increased macromolecular permeability accompanied by reorganization of apical actin and myosin. ZO-1 also participates in crosstalk with adherens junctions and cell–cell tension control, interacts with components such as VE‑cadherin and afadin, and engages signaling pathways that regulate cell migration, angiogenic behavior, and mechanosensitive transcriptional networks, including YAP-related outputs described for junctional MAGUKs. Dysregulated ZO-1 expression or mislocalization associates with epithelial barrier breakdown, inflammatory and fibrotic conditions, and carcinoma progression, where changes in tight junction integrity and junctional signaling influence invasion, permeability, and response to microenvironmental cues. |
