The tight junction marker occludin is a transmembrane protein interacting, for instance, with the scaffold ZO-1. On the molecular level, indications were given that occludin dimerizes and is sensitive to alterations in the redox status. Occludin is involved in the tight junction assembly and is of regulatory relevance for it. However, its physiological and pathological function is unclear.

Question: 

We therefore investigated whether oligomerization of occludin as well as its interaction with other tight junction proteins is redox-dependent in the cell.

Methods and Results: 

The homophiliccis- andtrans-interaction, measured by FRET and transcellular scanning, as well as strand formation, visualized by freeze-fracture electron microscopy, of human occludin was found attenuated during hypoxia or by C409A replacement at the regulatory fore-sequence of the coiled coil-domain of occludin. Similarly, occludin's association with ZO-1 at the cell membrane as well as the membranal localization of the latter is affected, as shown by live-cell imaging. The data demonstrate that disulfide bridge formation at the position 409 of the cytosolic coiled coil-domain in occludin is involved in its homologous oligomerization and the heterologous interaction with ZO-1 in the living cell. The model provided by molecular modeling suggests a heterotetramer as basic module for the polymerization of occludin in tight junction strands via ZO-1, the recruiting protein of the junctions.

Conclusion: 

The results support the new concept that occludin regulates the tight junction assembly in a redox-sensitive manner under physiological and pathological conditions.