1. Z.-H. Zeng, B. W. Segelke, E. A. Stura, and I. A. Wilson are in the Department of Molecular Biology and the Skaggs Institute for Chemical Biology at the Scripps Research Institute, La Jolla, CA 92037, USA. A. R. Castaño and P. A. Peterson are at the R. W. Johnson Pharmaceutical Research Institute at the Scripps Research Institute, La Jolla, CA 92037, USA.