1. Structural Biology Laboratory, Salk Institute for Biological Studies, La Jolla, California 92037
2. Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92037
3. Plant Biology Division, Samuel Roberts Noble Foundation, Ardmore, Oklahoma 73401
4. Institut de Biologie Structurale, 38027 Grenoble Cedex 1, France