Communication
The Structure of a Filamentous Bacteriophage

https://doi.org/10.1016/j.jmb.2006.06.027Get rights and content

Abstract

Many thin helical polymers, including bacterial pili and filamentous bacteriophage, have been seen as refractory to high-resolution studies by electron microscopy. Studies of the quaternary structure of such filaments have depended upon techniques such as modeling or X-ray fiber diffraction, given that direct visualization of the subunit organization has not been possible. We report the first image reconstruction of a filamentous virus, bacteriophage fd, by cryoelectron microscopy. Although these thin (∼ 70 Å in diameter) rather featureless filaments scatter weakly, we have been able to achieve a nominal resolution of ∼8 Å using an iterative helical reconstruction procedure. We show that two different conformations of the virus exist, and that in both states the subunits are packed differently than in conflicting models previously proposed on the basis of X-ray fiber diffraction or solid-state NMR studies. A significant fraction of the population of wild-type fd is either disordered or in multiple conformational states, while in the presence of the Y21M mutation, this heterogeneity is greatly reduced, consistent with previous observations. These results show that new computational approaches to helical reconstruction can greatly extend the ability to visualize heterogeneous protein polymers at a reasonably high resolution.

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Data Bank accession number

The coordinates of our model are deposited with the Protein Data Bank with accession number 2HI5.

Acknowledgements

This work was supported by NIH grants GM035269 and EB001567 (to E.H.E.) and GM50776 (to G.J.T.). We thank Drs Daniel Nemecek and James M. Benevides for helpful comments.

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