1. Department of Molecular Biology and the Skaggs Institute for Chemical Biology, The Scripps Research Institute, La Jolla, CA 92037, USA.
2. Institute for Protein Research, Pushchino, Russia.
3. Department of Molecular Biology, The Scripps Research Institute, La Jolla, CA 92037, USA.
4. Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.