Identification of a 349-kilodalton protein (Gli349) responsible for cytadherence and glass binding during gliding of Mycoplasma mobile

Several mycoplasma species are known to glide in the direction of the membrane protrusion (head-like structure), but the mechanism underlying this movement is entirely unknown. To identify proteins involved in the gliding mechanism, protein fractions of Mycoplasma mobile were analyzed for 10 gliding mutants isolated previously. One large protein (Gli349) was observed to be missing in a mutant m13 deficient in hemadsorption and glass binding. The predicted amino acid sequence indicated a 348,758-Da protein that was truncated at amino acid residue 1257 in the mutant. Immunofluorescence microscopy with a monoclonal antibody showed that Gli349 is localized at the head-like protrusion's base, which we designated the cell neck, and immunoelectron microscopy established that the Gli349 molecules are distributed all around this neck. The number of Gli349 molecules on a cell was estimated by immunoblot analysis to be 450 +/- 200. The antibody inhibited both the hemadsorption and glass binding of M. mobile. When the antibody was used to treat gliding mycoplasmas, the gliding speed and the extent of glass binding were inhibited to similar extents depending on the concentration of the antibody. This suggested that the Gli349 molecule is involved not only in glass binding for gliding but also in movement. To explain the present results, a model for the mechanical cycle of gliding is discussed.