Bacterial "gap junctions" in cell-cell communication of multicellular cyanobacteria

Cell-to-cell adhesion and communication are fundamental for the functioning of multicellular organisms. The most highly organized multicellular bacteria are found in the phylum cyanobacteria, where filaments may consist of hundreds of tightly interconnected cells. Cyanobacteria like Nostoc sp. differentiate heterocysts, specialized N₂-fixing cells, in a semi-regular pattern along the photosynthetic active filament. This life style requires a system of cell-cell communication and metabolite exchange by crossing the septal barrier thru cell-cell connections, the septal junctions. Our laboratory discovered in the species Nostoc punctiforme and Anabaena sp. PCC 7120 a puncturing of the septal peptidoglycan, the nanopore array, that is essential for multi-cellularity and molecule transfer. Cell wall lytic enzymes of the AmiC-type amidases are drilling the nanopores, and are regulated by a LytM factor. By focused ion beam (FIB) milling and electron cryotomography we could visualize the septal junctions, which connect adjacent cells and pass thru the nanopores. We identified the septal protein FraD as structural domain of the multiprotein complex. The septal junctions consist of a tube traversing the nanopore, a FraD-containing plug and a cytoplasmic cap. We also showed that the septal junctions are gated and close upon stress by reversible conformation changes in the cap structure. Our data thus provide a mechanistic framework for an ancient cell-cell connection structure, predating metazoan gap junctions by more than a billion years.