YANKING THE CHAIN: PULLING ON BACTERIAL FLAGELLA TO TRIGGER POLYMORPHIC TRANSFORMATIONS
March 6th, 2007
Authors Howard C. Berg & Nicholas C. Darnton
In this paper, we use an optical trap to apply forces comparable to those a filament would experience during swimming. From force-extension curves obtained from individual reconstituted, fluorescently labeled Salmonella filaments, we identify the forces required to cause polymorphic transformations. Within a single polymorphic form, the filament acts as a simple elastic object with a stiffness of 3.5 pN-µm2 Under extension, the filament transforms to a different, longer form. The transformation occurs in successive micron-long sections, at progressively lower forces. The force required depends on strain rate as well as on strain, suggesting there is a rate associated with the transformation, consistent with an activation phenomenon. Since the filament is a uniform polymer of flagellin protein, whose structure is known, it provides a simple, macroscopically visible model of highly cooperative conformational changes in a biological polymer.