Parts exchange: tuning the flagellar motor to fit the conditions.

Many cellular activities are driven by complex protein machines. By measuring the behaviour of fluorescent protein fusions in real time in living cells it has become apparent that many of these complexes are not fixed, but are dynamic. To some extent this might be expected, for example, for cell division complexes, as defining mid-cell is linked to growth and cell cycle, but perhaps comes as more of a surprise with a complex anchored machine like the bacterial flagellar motor. The assumption has been that once made it remains intact. However, the dynamics of this structure is strongly supported in two manuscripts in this issue of Molecular Microbiology. The stator units which form a peptioglycan anchored ring around the rotor, generating torque in response to the ion motive force, clearly disengage when conditions change. The driving ion is shown to be important in both engagement of the stator to the rotor and the selection of the type of stator unit. These new results provide an insight into the mechanisms underlying motor function, which might rely on dynamic processes, and clearly illustrate the need to move away from a static view of cellular structures.