1. Cell Biology and Metabolism Program, Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, MD 20892, USA.
2. Janelia Farm Research Campus, Howard Hughes Medical Institute, Ashburn, VA 20147, USA.
3. Virus-Cell Interaction Section, HIV Drug Resistance Program, Center for Cancer Research, National Cancer Institute, Frederick National Laboratory for Cancer Research, Frederick, MD 21702, USA.