1. Department of Pathology and Laboratory Medicine, University of Texas–Houston Medical School, Houston, TX 77030, USA.
2. National Center for Macromolecular Imaging, Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, TX 77030, USA.
3. Graduate Program in Structural and Computational Biology and Molecular Biophysics, Baylor College of Medicine, Houston, TX 77030, USA.
4. MRC Virology Unit, Institute of Virology, Glasgow G11 5JR, Scotland, UK.