1. Virologie Moléculaire & Structurale, UMR 2472/1157 CNRS-INRA, 1 Avenue de la Terrasse, 91198 Gif-sur-Yvette Cedex, France
2. Department of Cell Biology, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, New York 10461, USA
3. Present address: AFMB, CNRS UMR 6098, 31 Chemin Joseph Aiguier, 13402 Marseilles, France

Correspondence to: Margaret Kielian²Félix A. Rey¹ Email: rey@vms.cnrs-gif.fr
Email: kielian@aecom.yu.edu
The atomic coordinates of the E1* trimer, as well as the observed structure factors, have been deposited in the Protein Data Bank under accession code 1RER.

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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