1. Howard Hughes Medical Institute, The Jack H. Skirball Center for Chemical Biology and Proteomics, The Salk Institute for Biological Studies, La Jolla, California 92037
2. The Robert H. Smith Institute of Plant Sciences and Genetics in Agriculture, Faculty of Agriculture, The Hebrew University of Jerusalem, Rehovot 76100, Israel
3. Department of Molecular, Cellular, and Developmental Biology, University of Michigan, Ann Arbor, Michigan 48109