Membrane Complexes Required for Gliding Motility
Studies of the soil-dwelling bacterium Myxococcus xanthus have broadened our understanding of prokaryotic biology. M. xanthus uses gliding motility to forage for nutrients in relatively dry environments and to form a fruiting body containing quiescent spores in the absence of nutrients. We have discovered independent sets of genes encoding homologs of the E. coli transmembrane TolBQRA protein complex that are essential for gliding motility during growth. Surprisingly, the Tol proteins are not needed for gliding per se during development, because disruption of any of the tol genes does not affect aggregation or fruiting body formation. However, the spores produced by tol mutants fail to mature, showing that the Tol proteins are required at a late stage of spore differentiation. We will test the hypothesis that the Tol proteins are involved in the transport of biopolymers needed for gliding motility during growth and spore maturation during development. <br/><br/>Important clues to the function of the Tol complex have come from the study of one tol operon, the gidA- aglU (tolB1) operon. AglU, a homolog of TolB, is predicted to be an outer membrane lipoprotein. Like TolB, AglU contains WD-repeat motifs that form B-propellar structures that mediate protein-protein interactions with the TolQRA proteins located in the inner membrane. The gidA gene encodes a homolog of the E. coli glucose-inhibited division protein, a protein of unknown function present in many prokaryotes. Purified M. xanthus GidA absorbs at 450nm and has features showing that it is a flavoprotein. The gidA mutant does not aggregate and cells do not differentiate into spores. Moreover, the gidA mutant releases a substance that inhibits the development of wild-type cells. The inhibitor is likely exported via the Tol complex, because a second mutation in aglU prevents the release of this toxic substance. Analysis of the inhibitor, purified from the gidA mutant, may provide clues to the nature of the material secreted through the Tol complex.