1. Department of Life Science, Gwangju Institute of Science and Technology, Gwangju 500-712, Korea, Department of Bio-Materials, Graduate School and Research Center for Proteineous Materials, Chosun University, Gwangju 501-759, Korea, and Molecular Physiology and Biophysics Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland 20892