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

In Myxococcus xanthus, a predatory soil bacterium, cell polarity and motility are regulated by MglA (a small GTPase) and its regulators, MglB (a potential GAP) and RomRX (a previously proposed GEF), but their precise roles remain unclear. Using fluorescent nucleotides suitable for fluorescence anisotropy measurements, we show that RomRX does not function as a GEF but instead acts as an effector binding MglA_GTP. We further find, using an enzymatic model, that MglB exhibits both GAP and GEF properties depending on the local MglA_GDP concentration. Additionally, RomRX, specifically RomR, effectively dissociates the MglA-MglB complex by sequestering MglB. This low-affinity interaction permits GAP activity while still partitioning the proteins in vivo, as shown by a biomimetic oil-emulsion droplet assay in a cell-sized system. A minimal mathematical model incorporating these interactions and previous findings recapitulates how dynamic interactions between these three players form an invertible polarity axis in the M. xanthus cell.