Unique structural features of claudin-5 and claudin-15 lead to functionally distinct tight junction strand architecture.

Members of the claudin family impart unique paracellular selectivity to tight junctions. However, the structure-function relationship between claudin's strand architecture and the paracellular charge- and size-selectivity is not well-understood. This work examines the molecular assembly of claudin-5, a barrier-forming protein, and claudin-15, a channel-forming protein, to determine their structural and functional properties. We adopt a bottom-up approach starting from claudin monomers to build the molecular architecture of the tight junction strands. First, we investigated the cis assembly of claudin-5 and -15 dimers using the Protein Association Energy Landscape method. Out of the millions of dimer conformations, we narrowed down key cis claudin-5 and -15 dimer conformations that were thermodynamically and kinetically stable. Second, we performed the trans assembly of dimers to identify the tetrameric building blocks that serve as the repeat units for strand formation. Finally, the strand assembly of the tetrameric repeat units showed fundamentally distinct strand architectures. In claudin-5, the cis and trans interactions seal the paracellular space, while in claudin-15, the gaps in the paracellular space lead to pore formation. This detailed study suggests that each member of the claudin family is unique and requires systematic molecular-level analysis for determining the strand architecture.