Cell polarity control by an unconventional G-protein complex in bacteria

Abstract

In Myxococcus xanthus, cell polarity and motility are regulated by small GTPase MglA and its regulators, MglB, a potential GTPase Activating Protein and the RomRX complex, a potential Guanine nucleotide Exchange Factor. However, recent results have questioned the function of these regulators. Using fluorescent nucleotides for fluorescence-based anisotropy, we first demonstrate that RomRX does not function as a GEF but as an effector that binds strictly to MglA-GTP. Secondly, based on an enzymology model, we found that MglB functions as a weak catalyst, unlike typical GAPs, and requires additional factors to dissociate from MglA. We demonstrate that RomRX, specifically RomR, effectively competes with MglA by sequestering MglB. This low-affinity interaction is permissive for GAP activity but sufficient to partition these proteins in vivo, as indicated by a biomimetic assay in oil emulsion droplets reconstituting these interactions in a cell-sized environment. These results suggest a new model for cell polarity where RomRX exerts a dual function, targeting MglA-GTP to the pole via its effector function and preventing its accumulation at the lagging pole by destabilizing the MglA-MglB complex.