Structure and function of a bacterial gap junction analog

Multicellular lifestyle requires cell-cell connections. In multicellular cyanobacteria, septal junctions enable molecular exchange between sister cells and are required for cellular differentiation. The structure of septal junctions is poorly understood and it is unknown whether they regulate intercellular communication. Here we resolved the in situ architecture of septal junctions by electron cryotomography of cryo-focused ion beam-milled cyanobacteria. Septal junctions consisted of a tube traversing the septal peptidoglycan. Each tube end comprised a plug that was covered by a cytoplasmic cap. Fluorescence recovery after photobleaching showed that intercellular communication was blocked upon stress. This gating was accompanied by a conformational change of the septal junctions, mediated by the proteins FraC/D. We provide the mechanistic framework for a cell junction that predates eukaryotic gap junctions by a billion years. The conservation of a gated dynamic mechanism across different domains of life emphasizes the importance of controlling molecular exchange, e.g. upon injury.