The periplasmic and cytoplasmic faces of septal protein SepJ from filamentous cyanobacteria.

Filamentous, N-fixing, heterocyst-forming cyanobacteria grow as chains of cells in which intercellular transfer of regulators and metabolites takes place, allowing them to behave as multicellular organisms. Intercellular transfer occurs by diffusion through septal junctions. In the model heterocyst-forming cyanobacterium sp. strain PCC 7120, some identified septal proteins, including FraC and FraD, are directly involved in the formation of junctions that have been visualized by cryo-electron tomography, whereas the role of the key septal protein SepJ remains elusive. SepJ can form tetramers and contains coiled-coil, linker, and integral membrane (permease) domains. Using AlphaFold 3, a SepJ tetramer is predicted to have a quaternary structure in which the coiled-coil domain traverses the cytoplasmic membrane through a cavity formed between the four permease domains. Part of the coiled-coil domain is thus located in the septal periplasm, where it can interact with peptidoglycan. This possible SepJ structure can be widespread in filamentous cyanobacteria and explains known properties of SepJ. Structures of SepJ with other septal proteins including SjcF1, SepI, and SepT could also be predicted consistent with their previously described interactions. A possible interaction of the SepJ coiled-coil domain with the catalytic domain of cell wall amidase AmiC1, which would be relevant to prevent filament fragmentation in , is also discussed. The renewed view of SepJ presented here offers a molecular basis for understanding the key role of this protein in filament formation and intercellular communication.