1. Department of Biochemistry, Biophysics Instrumentation Facility, and National Magnetic Resonance Facility at Madison, University of Wisconsin−Madison, 433 Babcock Drive, Madison, Wisconsin 53706, Department of Biochemistry, Medical College of Wisconsin, 8701 Watertown Plank Road, Milwaukee, Wisconsin 53226, and Institute of Human Virology, University of Maryland, 725 West Lombard Street, Baltimore, Maryland 21201