Abstract
Filament-forming proteins in the bacterial cytoskeleton function in stabilization and localization of proteinaceous complexes and replicons. Research of the cyanobacterial cytoskeleton is focused on the bacterial tubulin (FtsZ) and actin (MreB). Nonetheless, the diverse colony morphologies and cell types in cyanobacteria suggest the presence of additional cytoskeletal proteins. Here we present two novel filament-forming proteins in cyanobacteria. Surveying cyanobacterial genomes for coiled-coil-rich proteins (CCRPs), we observed a higher proportion of CCRPs in filamentous cyanobacteria in comparison to unicellular cyanobacteria. We identified nine protein families with putative intermediate filament (IF) properties. Polymerization assays revealed four polymer-forming proteins in vitro and three polymer-forming proteins in vivo. Fm7001 from Fischerella muscicola PCC 7414 polymerized in vitro and formed filaments in different in vivo systems. Functional analysis of Fm7001 suggests that it has IF-like properties. Additionally, we identified a tetratricopeptide repeat protein, All4981 in Anabaena sp. PCC 7120 that polymerized into filaments in vivo and in vitro. All4981 interacts with other known cytoskeletal proteins and is indispensable for Anabaena. Our results expand the repertoire of known prokaryotic filament-forming CCRPs and demonstrate that cyanobacterial CCRPs are involved in cell morphology, motility, cytokinesis and colony integrity.
Author Summary The phylum Cyanobacteria is characterized by a large morphological diversity, ranging from coccoid or rod-shaped unicellular species to complex filamentous multicellular species. Many species of multicellular cyanobacteria can undergo cell differentiation and changes in their cell shape. Despite this diversity, very few molecular mechanisms underlying the cyanobacterial morphological plasticity are known. Among these, the cytoskeletal proteins FtsZ and MreB are important regulators of cyanobacterial cell shape and viability. Also, the multicellular phenotype of filamentous cyanobacteria has been linked to prokaryotic gap-junction analogs, the septal junctions. The significance of our research is the identification and characterization of a novel cyanobacterial cytoskeletal repertoire of IF-like proteins that will aid in the characterization of the morphological complexity of cyanobacteria. Thus, our survey leads to a broader understanding of the underlying principles of cyanobacterial morphotypes and will serve as a starting point for future research to further unravel the complex morphologies unique to this phylum.