Abstract

Bacterial flagellar filaments are the corkscrew-like propellers that enable bacteria such as Salmonella to swim in their aqueous environment. A flagellar filament is built, typically, from about 30,000 identical molecules of the protein flagellin. It has left-handed helical twist for normal swimming; but it transforms to a right-handed corkscrew when driven in reverse by its rotary motor during the “tumbling” manoever of chemotaxis.

Any helical form here is an engineering paradox: for if the building-blocks were in identical environments with respect to their neighbours, the filaments would have to be straight.

In the 1970s it was shown that, in different circumstances, flagellar filaments can adopt a range of different helical wave-forms, depending e.g. on mechanical torque (as above); pH of the environment; or single amino-acid mutations of the flagellin protein. But all of these different wave-forms were members of a single family of discrete forms – which included two extreme, straight varieties having Left- and Right-handed twist, respectively. In principle, this family could be explained by a quasi-mechanical conformational “switch” somewhere within the flagellin building-block.

In this talk I shall discuss two recent (2003, 2010) studies by Namba and colleagues, by means of cryo-electron-microscopy, of the L and R straight filaments of Salmonella at 4 Ǻ resolution. I shall use the difference between these two structures to elucidate not only the switching mechanism, but also the way in which a given flagellin mutant can determine which member of the family of discrete forms is to be constructed.

In building these helical flagellar filaments, nature makes use of helical structures at four different levels.