1. Department of Molecular Biology and Genetics, Cornell University, Ithaca, NY 14853
2. Weill Institute for Cell and Molecular Biology, Cornell University, Ithaca, NY 14853
3. Structural Studies Division, Medical Research Council Laboratory of Molecular Biology, Cambridge CB2 0QH, UK
4. Department of Cell and Virus Structure, Max Planck Institute of Biochemistry, Munich 82512, Germany