Notes

+These authors contributed equally to this work.

++Present address: Institut Curie, 26 Rue d'Ulm, 75248 Paris CEDEX 05, France.

+++These authors contributed equally to this work.

++++Present address: Plataforma Automatitzada de Cristal.lografía, Institute for Research in Biomedicine, Barcelona, Spain.

+++++These authors contributed equally to this work.

++++++Present address: Institut de Biologie Structurale Jean-Pierre Ebel, 41 Rue Jules Horowitz, 38027 Grenoble CEDEX 1, France

+++++++Present address: Institut Pasteur, Unité de Virologie Structurale, CNRS UMR 3569, 25 Rue du Docteur Roux, 75724 Paris CEDEX 15, France

1Supplementary material has been deposited in the IUCr electronic archive (Reference: MN5025 ). Services for accessing this material are described at the back of the journal.