1. Laboratório Nacional de Luz Síncrotron, P.O. Box 6192, Campinas SP 13084-971, Brazil, ImmunID, CEA/DRDC, 17 rue des Martyrs, 38057 Grenoble Cedex 9, France, and Unit of Virus Host Cell Interactions, UMR 5233, UJF-EMBL-CNRS, 6 rue Jules Horowitz, B.P. 181, 38042 Grenoble Cedex 9, France