1. Department of Chemical Physiology, The Scripps Research Institute, 10550 N. Torrey Pines Rd. La Jolla, California 92037, Whitehead Institute for Biomedical Research, Nine Cambridge Center, Cambridge, Massachusetts 02142, Howard Hughes Medical Institute, Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, Department of Biochemistry, University of Washington, Seattle, Washington 98195