Abstract
Bacteria, similar to eukaryotes, possess cytoskeletons that are involved in the temporal and spatial organization of various cellular processes, including cell division, cell morphogenesis, cell polarity, as well as DNA partitioning. Out of these elements, the tubulin homolog FtsZ, the actin homolog MreB, and intermediate filament-like (IF) proteins are widespread in many bacterial lineages. In addition, in recent years, an increasing number of non-canonical cytoskeletons have been identified in bacteria. These include a new class of cytoskeletal proteins, named bactofilins, which was originally discovered in Caulobacter crescentus. Bactofilins are widely distributed among bacteria and show no similarity in either sequence or structure to other known cytoskeletal proteins. Interestingly, many species possess two or more bactofilin alleles, indicating multiple gene duplication events and functional diversification. In Myxococcus xanthus, it has been shown that BacP, one of its four bactofilin homologs, forms bipolar filaments and appears to be essential for positioning a cell polarity factor, SofG; by constrast, BacM, another bactofilin homolog, is involved in cell morphogenesis. In this work, we demonstrate that BacP, together with two other bactofilin homologs, BacO and BacN, plays an important role in stabilizing the chromosome segregation machinery in this organism. We show that BacN-P copolymerize into bipolar filaments to mediate the proper arrangement of ParA and ParB, which are key components of chromosome segregation in M. xanthus. In the absence of BacN-P, both proteins mislocalize, which further affects proper chromosome segregation. We further identified BadA, a ParB-like nuclease homolog that acts along with BacN-P in this mechanism, possibly by interacting with ParA directly. Taken together, bactofilins serve as landmark structures at the cell poles in M. xanthus, positioning and stabilizing subpolar or polar protein complexes. This scaffolding function is similar to that of PopZ in C. crescentus or DivIVA in Gram-positive bacteria, despite the lack of sequence similarity among proteins. This similarity revealed a common theme in cell pole organization: landmark proteins form a higher-order structure that serves as an assembly platform for other proteins, thereby mediating their polar localization.
