[1] European Mol Biol Lab, Struct Biol Programme, D-69117 Heidelberg, Germany
[2] Univ Helsinki, Dept Biosci, FIN-00014 Helsinki, Finland
[3] Univ Helsinki, Inst Biotechnol, FIN-00014 Helsinki, Finland
[4] Publ Hlth Res Inst City New York Inc, Dept Microbiol, New York, NY 10016 USA
[5] Univ Oxford, Div Struct Biol, Wellcome Trust Ctr Human Genet, Oxford OX3 7BN, England