Myxococcus xanthus fibril appendages are essential for excitation by a phospholipid attractant

Isolated (A-motile) Myxococcus xanthus cells glide over solid surfaces and display excitation, a suppression of direction reversals, when presented wi th phosphatidylethanolamine (PE) purified from its own membranes or synthet ic dilauroyl PE and dioleoyl PE, Although the mechanism of PE signal transd uction is unknown, we hypothesized that M. xanthus might use surface-associ ated factors to detect exogenous PE to prevent endogenous lipids from self- stimulating the sensory system. Peritrichous protein and polysaccharide app endages called fibrils were correlated with dilauroyl PE excitation. Wild-t ype cells treated with Congo red, an inhibitor of fibril assembly, and muta nts defective in fibril biosynthesis showed an elevated reversal period, wh ich suggested that fibrils regulate the gliding motor. Furthermore, the los s of fibrils resulted in loss of excitation to dilauroyl PE but not dioleoy l PE, Restoration of fibril production to these mutants restored the dilaur oyl PE response. In addition, the dif cytoplasmic signal transduction syste m and starvation conditions were required for dilauroyl PE excitation. The chemically specific nature of the response and the dependence on the dif sy stem suggests that fibrils define a novel sensory organelle whose evolution may have been necessary to prevent autostimulation by endogenous membrane lipids. Because the hydrophobic nature of dilauroyl PE would be inaccessibl e to periplasmic chemosensors. we suggest that fibrils act as extracellular signal transducers to probe surfaces for insoluble chemical signals.