1. Department of Biology, Texas A&M University, College Station, Texas 778431;
2. Department of Biochemistry, University of Oxford, Oxford OX1 3QU, United Kingdom2;
3. Department of Molecular and Experimental Medicine, Scripps Research Institute, La Jolla, California 920373; and
4. Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut 065114