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Conformational change and protein–protein interactions of the fusion protein of Semliki Forest virus

Abstract

Fusion of biological membranes is mediated by specific lipid-interacting proteins that induce the formation and expansion of an initial fusion pore. Here we report the crystal structure of the ectodomain of the Semliki Forest virus fusion glycoprotein E1 in its low-pH-induced trimeric form. E1 adopts a folded-back conformation that, in the final post-fusion form of the full-length protein, would bring the fusion peptide loop and the transmembrane anchor to the same end of a stable protein rod. The observed conformation of the fusion peptide loop is compatible with interactions only with the outer leaflet of the lipid bilayer. Crystal contacts between fusion peptide loops of adjacent E1 trimers, together with electron microscopy observations, suggest that in an early step of membrane fusion, an intermediate assembly of five trimers creates two opposing nipple-like deformations in the viral and target membranes, leading to formation of the fusion pore.

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Acknowledgements

We thank S. Bressanelli, S. Duquerroy and P. Fernandez Varela for their help at different stages of this work; A. Ahn and A. Urian for help with virus and protein preparation; C. Schulze-Briese and T. Tomikazi for help during diffraction data collection; Y. Gaudin for critically reading the manuscript; and J. Navaza for helpful discussions. More than 80% of the data used to determine the crystal structure were collected at synchrotron beam line X06SA of the Swiss Light Source, Paul Scherrer Institut, Villigen, Switzerland. Other synchrotron sources used were beam lines ID14 and ID29 at the European Synchrotron Radiation Facility, Grenoble, France, and beam line BW7A at DESY, Hamburg, Germany. M.K. acknowledges support from the Public Health Service and from a Cancer Center Core Support Grant from the National Cancer Institute. F.A.R. acknowledges support from the CNRS and INRA, the SESAME Program of the Région Ile-de-France, the French Fondation pour la Recherche Médicale, the Association pour la Recherche contre le Cancer, the CNRS programs “Physique et Chimie du Vivant” and “Dynamique et réactivité des assemblages biologiques”, and the European Union ENhcV consortium. D.L.G. was supported by the Medical Scientist Training Program of the Albert Einstein College of Medicine, the Albert Cass Traveling Fellowship and the CNRS.Authors' contributions The crystallographic analyses reported in this paper were performed by F.A.R. and M.C.V.

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Correspondence to Margaret Kielian or Félix A. Rey.

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Figure 1: Overall fold of glycoprotein E1.
Figure 2: Trimer–trimer interactions observed in the 2D lattice and in the 3D crystals.
Figure 3: A ring of five trimers of E1*.
Figure 4: Model for membrane fusion involving protein–protein interactions, as explained in the text.

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