1. Department of Microbiology and Molecular Genetics, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania, USA
2. Pittsburgh Center for HIV Protein Interactions, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania, USA
3. Department of Structural Biology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania, USA
4. Department of Cancer Immunology and Virology, Dana-Farber Cancer Institute, Boston, Massachusetts, USA
5. Department of Medicine, Harvard Medical School, Boston, Massachusetts, USA
6. Department of Cell Biology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania, USA
7. Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, Pennsylvania, USA
8. Division of Structural Biology, Wellcome Trust Centre for Human Genetics, University of Oxford, Oxford, England
9. Electron Bio-Imaging Centre, Diamond Light Source, Harwell Science and Innovation Campus, Didcot, United Kingdom