Gated junctions in multicellular cyanobacteria

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Cell-to-cell communication is a prerequisite for the functioning of a multicellular organism. The most highly organized multicellular bacteria are found in the phylum cyanobacteria, where filaments may consist of hundreds of tightly interconnected cells. In the filamentous cyanobacterium Anabaena sp. PCC 7120, heterocysts differentiate from vegetative cells in a semi-regular pattern and are specialized in nitrogen fixation. Both cell types fulfill specific metabolic tasks and are interconnected in a homeostatic network of the entire multicellular filament. Transport of molecules and cell-cell communication along the filament occurs through the septal cell wall by multimeric protein structures, the septal junctions. In previous studies we have identified a nanopore array in the septal peptidoglycan and recently we solved the in situ architecture of septal junctions by electron cryo tomography (ECT) of ion beam milled filaments. These multiprotein complexes connect all cells of the filaments reaching from cell to cell thru the nanopores. The septal junctions consist of a tube traversing the septal peptidoglycan inside the nanopores. Each tube end comprises a plug inside the cytoplasmic membrane, which contains the FraD protein. The plug is covered by a cap structure with 5-fold symmetry on the cytosolic side. Fluorescence recovery after photobleaching showed that cell-cell communication was blocked upon stress in a reversible manner. This gating was accompanied by a reversible conformational change in the cap. In this project we want to continue our research on these bacterial gap junction analogs in close cooperation with Martin Pilhofer at ETH, Zürich, who will perform the ECT analysis. We want to know the composition and mechanism of these gated channels, which functional resemble metazoan gap junctions. For this we will identify the still unknown septal junction components using different approaches and investigate their individual function by mutagenesis, protein localization and biochemical studies. In addition we want to elucidate the physiological significance of the gating for the multicellular life style of these cyanobacteria and their adaptation to changing environmental conditions and predation.