1. School of Biological, Biomedical, and Molecular Sciences, The University of New England, Armidale, New South Wales 2351, Australia, Max-Planck-Institut fuer Biophysikalische Chemie, Abteilung Zellulaere Biochemie/Roentgenkristallographie, D-37077 Goettingen, Germany, and Department of Cell and Molecular Biology, John A. Burns School of Medicine, University of Hawaii at Manoa, Honolulu, Hawaii 96822