Protein interactions at the tight junction. Actin has multiple binding partners, and ZO-1 forms independent complexes with ZO-2 and ZO-3.

Defining how the molecular constituents of the tight junction interact is a prerequisite to understanding tight junction physiology. We utilized in vitro binding assays with purified recombinant proteins and immunoprecipitation analyses to define interactions between ZO-1, ZO-2, ZO-3, occludin, and the actin cytoskeleton. Actin cosedimentation studies showed that ZO-2, ZO-3, and occludin all interact directly with F-actin in vitro, indicating that actin is engaged in multiple interactions at the tight junction. Low speed sedimentation analyses demonstrated that neither ZO-2, ZO-3, nor occludin act as F-actin cross-linking proteins, and further evidence indicates that these proteins do not bind to actin filament ends. The binding interactions of ZO-2, ZO-3, and occludin were corroborated in vivo by immunofluorescence colocalization experiments which showed that all three proteins colocalized with actin aggregates at cell borders in cytochalasin D-treated Madin-Darby canine kidney cells. Exploration of other tight junction protein interactions demonstrated that ZO-2 binds directly to both ZO-1 and occludin. Contrary to previous beliefs, our immunoprecipitation results indicate that ZO-1, ZO-2, and ZO-3 exist in situ primarily as independent ZO-1.ZO-2 and ZO-1.ZO-3 complexes rather than a trimeric ZO-1.ZO-2.ZO-3 grouping. These studies elucidate direct binding interactions among tight junction-associated proteins, giving insight into their organization as a multimolecular structure.