1. Departments of Biopharmaceutical Sciences and Pharmaceutical Chemistry, and California Institute for Quantitative Biosciences, University of California at San Francisco, California 94158-2330;
2. Present addresses: Molecular and Computational Biology, Department of Biological Sciences, University of Southern California, Los Angeles, California 90089-2910;
3. Informatics Institute and Department of Computer Science, University of Missouri at Columbia, Missouri 65211;
4. School of Crystallography, Birkbeck College, University of London, London WC1E 7HX, United Kingdom;