1. Department of Microbiology, Immunology and Molecular Genetics, Molecular Biology Institute, University of California, Los Angeles, Los Angeles, CA 90095, USA.
2. Protein Sciences Department, Amgen Inc., Thousand Oaks, CA 91320, USA.
3. Institut de Génétique et de Biologie Moléculaire et Cellulaire, Illkirch cedex 67404, France.
4. Department of Cell and Structural Biology, University of Illinois, Urbana-Champaign, Urbana, IL 61801, USA.