Spatial Overlap of Claudin- and Phosphatidylinositol Phosphate-Binding Sites on the First PDZ Domain of Zonula Occludens 1 Studied by NMR.

Background: The tight junction is an intercellular adhesion complex composed of claudins (CLDs), occludin, and the scaffolding proteins zonula occludens 1 (ZO-1) and its two paralogs ZO-2 and ZO-3. ZO-1 is a multifunctional protein that contains three PSD95/Discs large/ZO-1(PDZ) domains. A key functional domain of ZO-1 is the first PDZ domain (ZO-1(PDZ1)) that recognizes the conserved C-termini of CLDs. Methods: In this study, we confirmed that phosphoinositides bound directly to ZO-1(PDZ1) by biochemical and solution NMR experiments. We further determined the solution structure of mouse ZO-1(PDZ1) by NMR and mapped the phosphoinositide binding site onto its molecular surface. Results: The phosphoinositide binding site was spatially overlapped with the CLD-binding site of ZO-1(PDZ1). Accordingly, inositol-hexaphosphate (phytic acid), an analog of the phosphoinositide head group, competed with ZO-1(PDZ)-CLD interaction. Conclusions: The results suggested that the PDZ domain⁻phosphoinositide interaction plays a regulatory role in biogenesis and homeostasis of the tight junction.