Identification and localization of the Tgl protein, which is required for Myxococcus xanthus social motility

Abstract

Tgl protein is required for the production of the type IV pili found at a pole of the Myxococcus xanthus cell. These pili are essential for social motility. Evidence is presented that Tgl is a membrane protein, based on experiments with polyclonal antibody specific for Tgl that was raised against the fusion proteins beta-galactosidase-Tgl and TrpE-Tgl. Immunoaffiity-purified antibody reacted with a protein in M. xanthus having an apparent molecular mass of 27.5 kDa as measured by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, while the sequence of the tgl gene translates into a polypeptide of 27 kDa. Although these numbers are close, it is likely that the primary tgl translation product is processed and modified in M. xanthus. The N terminus has a signal peptidase II recognition sequence, cleavage of which is expected to remove 19 amino acid residues. When the tgl gene is expressed in Escherichia coli, the protein product consistently migrates faster in the gel than mature Tgl expressed in M. xanthus, suggesting a second modification by addition which slows migration of the protein from M. xanthus. Tgl, as detected by its specific antibody, sediments with the membrane fraction of cells. It can be extracted with detergents but not with salt or by the addition of chelators for divalent cations. In an equilibrium gradient, Tgl bands at the buoyant density of membranes and with the NADH-oxidase activity. Intact cells failed to bind anti-Tgl antibody, and less than 2% of the total Tgl is released in soluble form from the periplasm. Yet, cells that had been osmotically shocked and treated with paraformaldehyde were able to react with the specific antibody--a reaction absent from cells with a deletion of the tgl transcription unit. Assuming that osmotic shock disrupts the outer membrane, the fractionation and localization data imply that Tgl is attached to the inner or outer membranes, from which it is exposed to the intermembranous space. Tgl is necessary for synthesis of pili in M. xanthus and is the only pilus protein that can be donated by other cells (stimulation). Tgl contains six tandem copies of the tetratrico peptide repeat structural motif. Its membrane localization, capacity for stimulation, and content of tetratrico structural repeats together suggest that Tgl may be necessary for the assembly of pilin subunits into filaments.