The SepN-FraD structural module serves as an anchor and assembly platform for septal junctions in cyanobacteria

Intercellular communication is essential for multicellularity. In filamentous cyanobacteria such as Nostoc sp. PCC 7120, communication occurs through septal junctions (SJs) that traverse the septum and connect neighboring cells. Although the SJ components FraD and SepN are essential for SJ formation, their precise roles and spatial organization remain unclear. Here, we combine cryo-electron tomography, AlphaFold 3 structure prediction, and molecular dynamics simulation to refine the SJ architecture. We identify previously unresolved structural features, including a continuous, membrane-like tube connecting septal junction heads of neighboring cells and membrane-anchoring modules linking the plug to the periplasmic space. Structural modeling positions SepN as the plug-forming component and FraD as a membrane-spanning anchor with a periplasmic domain. Functional analyses demonstrate that the FraD periplasmic domain is required for proper nanopore formation, SJ assembly, and diazotrophic growth. Together, these findings establish the SepN-FraD module as the structural scaffold essential for SJ integrity and gating.