Mycoplasma takes a walk.

T he mechanism of movement of individual cells shows extensive diversity. In the Eukarya, ATP is the driving force for cell motility (1). In contrast, in the Bacteria and Archaea, the chemiosmotic pathway of energy transduction powers flagellar rotation. Depending on the species, either hydrogen or sodium ion flux results in cell movement. ATP does not directly participate in providing the energy needed for this type of motility (2). But some species of bacteria move by means other than flagellar-based motility. Do hydrogen or sodium gradients propel these organisms as well? In a recent issue of PNAS, Uenoyama and Miyata (3) directly showed that for a wall-less bacterial species, Mycoplasma mobile, intracellular ATP is the driving force for its relatively fast gliding along a glass surface. This is the second type of motility powered by ATP identified in bacteria, the first being the retraction of type IV pili as seen, for example, in Myxobacteria and Pseudomonas (4, 5). Uenoyama and Miyata (3) used a multistep approach to reach this conclusion, but the essential experimental design was borrowed from eukaryotic cell biology. First, they isolated a spontaneously occurring mutant of M. mobile that can attach more efficiently to a glass surface than the wild type does. This mutant enabled them to assay the motility of a large number of cells. Second, using a procedure similar to that used by Gibbons and Gibbons (6) on sea urchin sperm, they rendered the membrane of gliding M. mobile cells permeable with a low concentration of the detergent Triton X-100. What remained were nonmotile ghosts that lacked DNA but still adhered to the glass. Finally, when incubated with ATP, the ghosts regained motility with speeds similar to untreated cells ( 2 m s). The results are striking, and the reader is urged to view the accompanying movie in their supporting information. Uenoyama and Miyata tested other nucleotide triphosphates, but ATP worked most efficiently. These results, plus other published experiments on intact cells, including one that showed that arsenate inhibited gliding of M. mobile (7), conclusively point toward ATP as the driving force. How do these bacteria glide, and how does ATP drive cell motion? Although much has yet to be worked out, over the past several years, significant progress has been made along these lines (8). M. mobile attaches to a glass surface, and it then aligns itself such that it tapers at one cell pole; the cell resembles a flask with a head-like structure, neck, and cell body (ref. 9 and Fig. 1 Upper). Its dimensions are quite small, with a long axis of only 1.0 m and a short axis of 0.6 m (10). Attachment to the glass and cell displacement occur at the neck, with the head serving as the leading end of the cell (11). The cell does not reverse directions, and it exerts a substantial force of 27 pN during gliding (12). Three major approaches have been used to identify cellular proteins involved in cell motility and attachment. First, because there is no known genetic exchange system for M. mobile, and thus no way to target specific genes, UVinduced mutants were isolated, screened, and characterized for ones that were deficient in either motility or attachment to a surface (10). Second, genes adjacent to others previously identified as being involved in gliding, and part of the same operon, were hypothesized to participate in motility. These genes, and the proteins that they encode, were analyzed in detail (13). Third, monoclonal antibodies were screened for their ability to inhibit mo-