1. Department of Biochemistry & Molecular Pharmacology, New York University School of Medicine, New York, NY 10016, U.S.A.
2. Division of Bioorganic Chemistry, Faculty of Chemistry, Wroclaw University of Technology, Wyb. Wyspianskiego 27, Wroclaw 50-370, Poland
3. Division of Cell Biology, Netherlands Cancer Institute (NKI), Plesmanlaan 121, 1066 CX, Amsterdam, The Netherlands
4. Structural Biology Program, Sloan-Kettering Institute, 1275 York Ave, New York, NY 10065, U.S.A.
5. Howard Hughes Medical Institute, 1275 York Ave, New York, NY 10065, U.S.A.