Cyanobacterial septal junction module

The septal junction (SJ) is a proteinaceous, gap-junction-like cell-cell junction of filamentous, heterocyst-forming cyanobacteria (e.g. Nostoc/Anabaena sp. PCC 7120). SJs traverse the shared septal peptidoglycan (PG) through nanopores and directly connect the cytoplasms of adjacent cells in a filament, mediating and gating the intercellular diffusion of small molecules (metabolites and signaling compounds) — a prerequisite for multicellular behavior and for diazotrophic growth, in which heterocysts exchange fixed nitrogen for photosynthate with neighboring vegetative cells. In situ cryo-electron tomography with subtomogram averaging resolves the SJ as a five-fold-symmetric assembly with four structural modules: a cytoplasmic cap (five arches), a membrane-embedded plug, a septum-spanning tube, and a transmembrane/periplasmic anchor. This module represents the SJ as a structural cell-cell junction: its two molecularly identified core components — SepN (the plug) and FraD (the membrane/periplasmic anchor) — the still molecularly-unidentified cap and (partly lipidic) tube modules, the cell-wall amidases that drill the septal PG nanopore array the SJ passes through, and the additional septal proteins (SepJ/FraG, FraC, FraE) that influence SJ architecture and filament integrity.

Scope and caveats. This module is centered on the cryoET-resolved SJ structural complex and its FraD-SepN core (the two components with a defined molecular role and per-gene reviews). The cap module's molecular identity is unknown (an open question in the field), and the tube appears to be a membrane-continuous, at-least-partly lipidic extension of the cytoplasmic membrane rather than a purely proteinaceous ring, so both are represented as abstract structural sub-modules without a grounded protein member. The septal PG amidases (AmiC1/AmiC2) act upstream to create the nanopores the SJ traverses and are modeled as an assembly prerequisite, not as SJ subunits. SepJ (FraG), FraC and FraE are septal-junction-associated proteins that influence SJ architecture, nanopore number and filament integrity; whether SepJ forms the same channel as the FraCD/SepN complex or a distinct but related junction is not fully resolved, so they are grouped as associated/architectural factors rather than asserted subunits of the FraD-SepN core. Stoichiometry of the core is 5 SepN + 5 FraD per SJ head (C5 symmetry). Grounding convention: each concrete leaf annoton is a FAMILY descriptor (grounded to its InterPro family term where one exists, or preferred_term + description for SepN and SepJ, which lack a dedicated family model), with the Nostoc/Anabaena sp. PCC 7120 (NCBITaxon:103690) protein(s) listed as representative_members exemplars rather than as the sole species-specific participant. The two paralogous septal amidases AmiC1 (alr0092) and AmiC2 (alr0093) are represented as a single AmiC-family annoton with both as exemplars. A reproducible genome scan validating these groundings against other cyanobacterial genomes (positive: Nostoc punctiforme, Anabaena variabilis; negative: unicellular Synechocystis/Synechococcus) lives with the module in modules/septal_junction/ (RESULTS.md; regenerate with `ai-gene-review scan-module modules/septal_junction.yaml --targets modules/septal_junction/scan_targets.json --homology`): FraD/FraC (specific families) and SepN (by exemplar homology, no family model) are cleanly present in heterocyst-formers and absent from unicellular controls, while SepJ/FraE/AmiC map to broad domains that also hit non-SJ paralogs.