SepN is essential for assembly and gating of septal junctions in Nostoc sp. PCC 7120

Summary The multicellular life style of filamentous cyanobacteria like Nostoc sp. PCC7120 relies on a cell-cell communication system involving so called septal junctions. These are multiprotein complexes, which traverse the septal peptidoglycan through nanopores, connecting the neighboring cells and enabling molecule transfer along the filament. The intercellular communication is crucial when different cell types in the filament, vegetative cells and heterocysts, have to exchange metabolites and signaling molecules. Septal junctions of cyanobacteria can even control the molecule exchange by gating. The multiprotein complex consists of three modules: the septum spanning tube, the plug residing within the cytoplasmic membrane at both ends of the tube and a membrane associated cap module, covering the plug/tube modules on the cytoplasmic side of each neighboring cell. Until now, FraD was the only identified protein component of the septal junction protein complexes and localizes to the plug module. Here, we identified SepN as a new component via co-immunoprecipitation using FraD as bait and further demonstrated its essential role in septal junction assembly. Despite normal septal nanopore formation, a mutant in sepN exhibited a highly reduced rate of intercellular communication and was unable to gate the exchange of molecules. Cryo-electron tomography of cryo-focused ion beam thinned sepN -mutant filaments revealed septal junctions lacking the plug module and lateral cap openings. The combination of missing plug but present cap allowed to deduce the importance of the plug module in ensuring the correct cap architecture and, more importantly, in sealing the diffusion area in the closed septal junction state.