Aggregation of Claudins and Formation of Tight Junctions: A Coarse-Grained Molecular Dynamics Study

Abstract

Tight junctions are dynamic structures that consist of a number of membrane proteins and their cytoplasmic counterparts. Claudins are one the major components of tight junctions that control the transport of ions and small molecules in paracellular pathways. The recently solved crystallographic structure of claudin-15 shows a membrane protein consisting of four transmembrane helices (TM1-TM4) and a large extracellular domain (ESC). Claudin monomers polymerize in membrane to form tight junction strands, which are connected to similar strands in neighboring cells and glue two cells together. We have used the structure of claudin-15 to investigate the polymerization and assembly of claudins in lipid membranes with molecular dynamics simulations. The simulations revealed that the linear arrangement of claudins as observed in the crystal is not stable in the membrane. Individual claudins rotate in the membrane to form new contacts between TM3 and ECS resulting in a curved configuration with a diameter of 100A. To obtain more insight about the side-by-side interaction of claudins in the membrane, 16 claudins were placed in a lipid bilayer in a 4x4 matrix arrangement and their diffusion was studied in coarse-grained simulations. In these simulations, claudins form linear configurations consisting of 2-8 claudins stabilized by interactions between TM2 or TM3 segments and ECS. These results show that the side-by-side interaction of claudins in the membrane occurs through two interfaces. The presence of two interaction surfaces might in fact be a key factor in stabilization of long and flexible tight junction strands in the cell. In addition, head-to-head interaction of claudins across two neighboring cells, which is essential for tight junction formation, was studied. In the presence of trans interactions, claudins show a slower dynamics in each membrane and form shorter strands.