1. Department of Structural Biology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15260, USA.
2. Pittsburgh Center for HIV Protein Interactions, University of Pittsburgh School of Medicine, Pittsburgh, PA 15260, USA.
3. MRC Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge CB2 0QH, UK.
4. Department of Physics and Beckman Institute, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.
5. Department of Cancer Immunology and Virology, Dana-Farber Cancer Institute, Boston, MA 02215, USA.
6. Department of Medicine, Harvard Medical School, Boston, MA 02115, USA.
7. Division of Structural Biology, Henry Wellcome Building for Genomic Medicine, University of Oxford, Headington, Oxford OX3 7BN, UK.
8. Electron Bio-Imaging Centre, Diamond Light Source, Harwell Science and Innovation Campus, Didcot OX11 0DE, UK.