1. Department of Biochemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, Structural Biology Center, Bioscience Division, Argonne National Laboratory, Argonne, Illinois 60439, Department of Biochemistry, Molecular Biology and Cell Biology, Northwestern University, Evanston, Illinois 60208, and Institute of Biological Chemistry, Academia Sinica, Taipei, Taiwan, Republic of China