1. Howard Hughes Medical Institute and Department of Biochemistry, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75390, USA.
2. Department of Chemistry and Biochemistry, University of Oklahoma, 620 Parrington Oval, Norman, OK 73019, USA.
3. Department of Health Sciences, College of Public and Allied Health, East Tennessee State University, Post Office Box 70673, Johnson City, TN 37614, USA.
4. Laboratory of Cell Biology, National Cancer Institute, National Institutes of Health, 37 Convent Drive, Bethesda, MD 20892, USA.
5. Department of Biochemistry and Microbiology, University of Victoria, Post Office Box 3055, Victoria, British Columbia V8W 3P6, Canada.