1. Laboratory of Molecular Biology, Department of Crystallography, Birkbeck College, London WC1E 7HX, United Kingdom.
2. Department of Chemistry, University of Arizona, Tuscon, AZ 85721.
3. Center for Polypeptide and Membrane Research, City University of New York, New York, NY 10029.
4. Department of Physiology and Biophysics, and Center for Polypeptide and Membrane Research, Mount Sinai School of Medicine of the City of New York, New York, NY 10029.