Force-extension measurements on bacterial flagella: triggering polymorphic transformations
Nicholas C Darnton 1 and Howard C Berg 1*
1 Harvard University
* To whom correspondence should be addressed. E-mail: hberg{at}biosun.harvard.edu.
Submitted on July 27, 2006
Revised on September 30, 2006
Accepted on 23 October 2006
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Abstract |
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Bacterial flagella can adopt several different helical shapes in response to varying environmental conditions. A geometrical model due to Calladine ascribes these discrete shape changes to cooperative transitions between two stable tertiary structures of the constituent protein, flagellin, in agreement with more recent physical data, and predicts an ordered set of 12 helical states called polymorphic forms. Using long polymers of purified flagellin, we demonstrate controlled, reversible transformations between different polymorphic forms. By pulling on a single filament using an optical tweezer, we record the progressive transformation of the filament and also measure the force-extension curve. Both normal and coiled polymorphic forms stretch elastically with a bending stiffness of 3.5 pN-µm2. At a force threshold of 4-7 pN or 3-5 pN (for normal and coiled forms, respectively), a fraction of the filament suddenly transforms to the next, longer, polymorphic form. This transformation is not deterministic, since the force and amount of transformation vary from pull to pull. In addition, the force is highly dependent on stretching rate, suggesting that polymorphic transformation is associated with an activation energy
Key Words:
Salmonella typhimurium, elasticity, flagellin, fluorescence, optical tweezer, protein structure
