1. Department of Biological Sciences, Lilly Hall of Life Sciences, Purdue University, West Lafayette, Indiana 47907-2054, Structural Biology and Microimaging, Battelle Northwest Laboratory, P.O. Box 999 K8-98, Richland, Washington 99352-0999, and Institute of Biological Chemistry, Washington State University, 289 Clark Hall, Pullman, Washington 99164-6340