1. Division of Biochemistry and Molecular Biology, John Curtin School of Medical Research, Australian National University, G.P.O. Box 334 Canberra, ACT, 2601, Australia, Department of Biochemistry and Biological NMR Centre, University of Leicester, Leicester LE1 9HN, U.K., and Department of Biochemistry, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China