1. Department of Biology, Washington University, St. Louis, MO 63130, USA.
2. European Synchrotron Radiation Facility, 38000 Grenoble, France.
3. European Molecular Biology Laboratory, Grenoble, France.
4. Unit of Virus Host-Cell Interactions, Université Joseph Fourier–European Molecular Biology Laboratory–CNRS, Grenoble, France.