[1] Univ London Imperial Coll Sci Technol & Med, Wolfson Labs, Dept Biol Sci, London SW7 2AZ, England
[2] Univ London Imperial Coll Sci Technol & Med, Ctr Struct Biol, London SW7 2AZ, England
[3] Inst Pasteur, Unite Pathogenie Bacterienne Muqueuses, F-75724 Paris 15, France