Dynamic behavior of paired claudin strands within apposing plasma membranes.

The tight junction (TJ) strand is a linear proteinaceous polymer spanning plasma membranes, and each TJ strand associates laterally with another TJ strand in the apposing membranes of adjacent cells to form "paired" TJ strands. Claudins have been identified as the major constituents of TJ strands, and when exogenously expressed in L fibroblasts, they polymerize into paired strands, which are morphologically similar to paired TJ strands in epithelia. Here, we show that a fusion protein of GFP with claudin-1 can also form similar paired strands in L fibroblasts, allowing us to directly observe individual paired claudin strands in live cells in real time. These paired strands showed more dynamic behavior than expected; they were occasionally broken and annealed, and dynamically associated with each other in both an end-to-side and side-to-side manner. Through this behavior of individual paired claudin strands, the network of strands was reorganized dynamically. Furthermore, fluorescence recovery after photobleaching analyses revealed that claudin molecules were not mobile within paired strands. Although these observations are not necessarily representative of TJ strands per se in epithelial cells, they provide important information on the structural and kinetic properties of TJ strands in situ with significant implications for barrier function of TJs.