1. Department of Chemistry and Graduate Programs in Biophysics and in Molecular and Cellular Biology, State University of New York at Stony Brook, Stony Brook, New York 11794-3400, Bristol-Myers Squibb Pharmaceutical Research Institute, P.O. Box 4000, Princeton, New Jersey 08543-4000, and Department of Molecular, Cell and Developmental Biology, Haverford College, Haverford, Pennsylvania 19041