1. Program in Biophysics, University of Michigan, Ann Arbor, Michigan 48109-1055, United States
2. Department of Microbiology and Molecular Genetics, The Institute for Medical Research, Hebrew University of Jerusalem, 91120 Jerusalem, Israel
3. Department of Molecular, Cell and Developmental Biology, University of Michigan, Ann Arbor, Michigan 48109-1048, United States
4. Department of Biological Chemistry, University of Michigan, Ann Arbor, Michigan 48109-1055, United States