1. Department of Structural Biology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15261, United States
2. Pittsburgh Center for HIV Protein Interactions, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15261, United States
3. Department of Biology, Johns Hopkins University, Baltimore, Maryland 21218, United States
4. Department of Cell Biology, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States
5. Computational Biomolecular Magnetic Resonance Core, Laboratory of Chemical Physics, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892-0520, United States