1. Institute for Cellular and Molecular Biology, Department of Chemistry and Biochemistry, University of Texas at Austin, Austin, TX 78712, USA.
2. MRC Laboratory for Molecular and Cell Biology and Cell Biology Unit, Department of Pharmacology, University College London, Gower Street, London WC1E 6BT, UK.
3. Howard Hughes Medical Institute, Departments of Medicine and Biochemistry, Duke University Medical Center, Durham, NC 27710, USA.