1. Graduate School of Pharmaceutical Sciences, Tohoku University, Sendai 980-8578, 2Graduate School of Biological Sciences, NAIST, Ikoma 630-0192, 3Graduate School of Medical Science, Kyoto Prefectural University of Medicine, Kyoto 603-8334, 4Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, Fukuoka 819-0395 and 5Institute for Protein Research, Osaka University, Suita 565-0871, Japan