A new family of bacterial actin-like proteins regulates cell morphology in a filamentous cyanobacterium

Actin proteins are common to all domains of life and exhibit ATP-dependent polymerization to form filaments. In bacteria, four families of bacterial actin-like proteins (BALPs) have been identified and characterized. These BALPs are involved in plasmid partitioning (ParM), cell division (FtsA), magnetosome positioning (MamK), and cell morphology (MreB). Here, we report the identification of a fifth family of BALP, FcmB. Using the model filamentous cyanobacterium <i>Nostoc punctiforme</i>, we demonstrate that FcmB is a BALP that regulates cell morphology in filamentous cyanobacteria. Deletion of <i>fcmB</i>, or <i>fcmC</i>, which encodes an FcmB-interacting protein, resulted in the loss of rod morphology, similar to the phenotype reported for <i>mreB</i> mutants in other bacteria, including cyanobacteria. However, despite the apparent functional similarity, <i>fcmB</i> is not a paralog of <i>mreB</i>, but rather was acquired by horizontal gene transfer of a plasmid partitioning system and subsequent integration into the chromosome. Fluorescent protein fusions and immunofluorescence demonstrate that FcmB forms membrane-bound filaments which wrap around the circumference of the cell, while FcmC is localized to discrete membrane-associated foci and is essential for proper membrane localization of FcmB. Protein-protein interactions were detected between FcmB and FcmC, but not MreB, indicating that FcmB and MreB do not form heterofilaments. It is currently unclear how FcmBC exerts its effect on cell morphology, but both <i>mreB</i> and <i>fcmB</i> are ubiquitous in the developmentally complex heterocyst-forming filamentous cyanobacteria, and the presence of two discrete systems modulating cell morphology may be critical for the remarkable degree of phenotypic plasticity observed in these organisms.IMPORTANCEFilament-forming actin proteins are found in nearly all living organisms. In bacteria, four families of actin proteins have been defined, with biological functions in plasmid partitioning, cell division, magnetosome positioning, and cell morphology. Here, we identify and characterize FcmB, a fifth family of bacterial actin proteins found in filamentous cyanobacteria, and demonstrate that this family evolved from plasmid partitioning actins but influences cell morphology rather than DNA segregation. Filamentous cyanobacteria exhibit substantial phenotypic plasticity and typically contain both FcmB and MreB, the other actin family known to regulate cell morphology. The presence of two distinct families of actin proteins influencing cell morphology may play a critical role in the ability of these organisms to rapidly alter their cell shape.