1. Canadian Institutes of Health Research Group in Molecular Biology of Membrane Proteins, University of Alberta, Edmonton, Alberta, Canada, T6G 2H7,
2. Department of Biochemistry, University of Alberta, Edmonton, Alberta, Canada, T6G 2H7,
3. La Jolla Institute for Allergy and Immunology, San Diego, California 92121,
4. Department of Anatomy and Cell Biology, University of Toronto, Toronto, Ontario, Canada M5S 1A8