Tight junctions (TJs) are dynamic cellular structures that are critical for compartmentalizing environments within tissues and regulating transport of small molecules, ions, and fluids. Phosphorylation-dependent binding of the transmembrane protein occludin to the structural organizing protein ZO-1 contributes to the regulation of barrier properties; however, the details of their interaction are controversial. Using small angle X-ray scattering (SAXS), NMR chemical shift perturbation, cross-saturation, in vitro binding, and site-directed mutagenesis experiments. we define the interface between the ZO-1 PDZ3-SH3-U5-GuK (PSG) and occludin coiled-coil (CC) domains. The interface is comprised of basic residues in PSG and an acidic region in CC. Complex formation is blocked by a peptide (REESEEYM) that corresponds to CC residues 468-475 and includes a previously uncharacterized phosphosite, with the phosphorylated version having a larger effect. Furthermore, mutation of E470 and E472 reduces cell border localization of occludin. Together, these results localize the interaction to an acidic region in CC and a predominantly basic helix V within the ZO-1 GuK domain. This model has important implications for the phosphorylation-dependent regulation of the occludin∶ZO-1 complex.membrane-associated guanylate kinase | tricellulin | calmodulin T ight junctions (TJs) are highly polarized gates that control the flux of fluids, proteins, and even ions across sheets of endothelial or epithelial cells (1, 2). These barriers function in a range of tissues, including the vasculature of the central nervous system, kidney, and gut epithelium. Dysregulation of barrier properties is associated with a host of disease states including cancer, stroke, diabetic retinopathy, and inflammatory bowel syndrome (3-5). Furthermore, viral and bacterial pathogens exploit specific TJ proteins to gain access to host cells. Thus, understanding the contribution of TJ components in barrier formation and regulation may aid the development of therapies to restore barrier properties, control barrier properties to promote drug delivery to regions of the CNS, and for the development of novel antibacterial and antiviral compounds.Occludin is an integral membrane, vesicle-trafficking, MAL and related proteins for vesicle trafficking and membrane link (MARVEL) protein (6), that has a role in regulating TJ properties (7). For example, in endothelial cells, it regulates TJ barriers in response to cytokines, such as IFN-γ (8, 9), and growth factors, such as VEGF (10). Removal of the cytoplasmic C-terminal coiled coil domain (CC; residues 413-522) of occludin leads to cytoplasmic localization with increased tracer flux through the junctions and an inability to maintain the apical localization of marker proteins (11,12), identifying the CC as a key element in regulation (13). The CC directly associates with the SH3-U5-GuK domains of the membrane-associated guanylate kinase homolog (MAGUK) protein, . ZO proteins are characterized by their core PDZ3-SH3-GuK (PSG) doma...