Septal protein FraI is a new player in cell-cell communication and cell differentiation in multicellular cyanobacteria

Filamentous cyanobacteria are prominent examples of bacterial multicellularity. Hundreds of photosynthetic cells are interconnected in a homeostatic network and are capable of cell differentiation in adaptation to changing environment ¹. Depending on species, dormant spore-like akinetes, N₂ fixing heterocysts and motile hormogonia are formed¹. To coordinate the physiological processes, these cyanobacteria exhibit a sophisticated cell-cell communication system ^2,3. The cellular basis of molecule exchange between the cells are septal junctions (SJ) ⁴, bridging the septum thru 20-nm wide nanopores in the peptidoglycan ^5,6. The SJ consist of 3 modules: the septum crossing tube, the plug residing in the cytoplasmic membrane and a cap on the cytoplasmic side ⁷. A mutant of Nostoc punctiforme ATCC 29133 lacking the cell wall amidase AmiC2 was not able to form the nanopores and to undergo cell differentiation ^4,5. Similarly, the homologues proteins AmiC1 and AmiC2 of Anabaena PCC 7120 have functions in nanopore and heterocyst formation ⁸.

So far, we identified two proteins of the SJ, FraD and SepN ^7,9. FraD mutants lack cap and plug; SepN mutants just the plug. Consequently, SJ gating, which allows closure of SJs in stress situations^7,9, is impaired and the filaments exhibit severe phenotypes, like fragmentation and aberrant heterocyst differentiation.

Here we present FraI, encoded by alr4714 (Anabaena PCC 1720) and NpF4142 (Nostoc punctiforme ATCC 29133), respectively. Mutants of both strains show defects in cell differentiation and diazotrophic growth, and almost no cell-cell communication. We show that this is due to the lack of nanopores and SJ, proving the important role of the nanopore array for the multicellular lifestyle of these cyanobacteria.

1 Maldener et al. 2014 in The Cell Biology of Cyanobacteria 239–304

2 Flores et al. 2019 Life 9

3 Nieves-Morión et al. 2017 mBio 8, e01756-16

4 Kieninger et al. 2021 Curr Opin Microbiol 61, 35–41

5 Lehner et al. 2013 FASEB Journal 27, 2293–2300.

6 Nürnberg et al. 2015 mBio 6

7 Weiss et al. 2019 Cell 178, 374-384.e15

8 Bornikoel et al. 2017 Front Cell Infect Microbiol 7

9 Kieninger et al. 2022 Nat Commun 13