Bacterial mobility is powered by rotation of helical flagellar filaments driven by rotary motors. Flagellin isolated from the Salmonella Typhimurium SJW1660 strain, which differs by a point mutation from the wild-type strain, assembles into straight filaments in which flagellin monomers are arranged in a left-handed helix. Using small-angle x-ray scattering and osmotic stress methods, we investigated the structure of SJW1660 flagellar filaments as well as the intermolecular forces that govern their assembly into dense hexagonal bundles. The scattering data were fitted to models, which took into account the atomic structure of the flagellin subunits. The analysis revealed the exact helical arrangement and the super-helical twist of the flagellin subunits within the filaments. Under osmotic stress, the filaments formed two-dimensional hexagonal bundles. Monte Carlo simulations and continuum theories were used to analyze the scattering data from hexagonal arrays, revealing how the bundle bulk modulus and the deflection length of filaments in the bundles depend on the applied osmotic stress. Scattering data from aligned flagellar bundles confirmed the theoretically predicated structure-factor scattering peak line shape. Quantitative analysis of the measured equation of state of the bundles revealed the contributions of electrostatic, hydration, and elastic interactions to the intermolecular forces associated with bundling of straight semi-flexible flagellar filaments.
Bibliographical noteFunding Information:
D.L., A.G., T.D., and U.R. acknowledge financial support from the Israel Science Foundation (grant 1372/13), US-Israel binational Science Foundation (grant 2009271), Rudin, Wolfson, and Safra Foundations, and the FTA-Hybrid Nanomaterials program of the Planning and Budgeting Committee of the Israel Council of Higher Education. D.L. and T.D. thank the Center for Nanoscience and Nanotechnology of the Hebrew University for fellowships. A.G. thanks the Institute for Drug Research at the Hebrew University for a fellowship. Z.D. and W.S. acknowledge support from the National Science Foundation through grants DMR-CMMI-1068566, NSF-DMR-1609742, and NSF-MRSEC-1420382. We also acknowledge use of the Materials Research Science and Engineering Center Biosynthesis facility, supported by grant NSF-MRSEC-1420382. Z.D., W.S., and U.R. acknowledge travel support from the Bronfman Foundation.
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