1. Cell Surface Receptor Group, Health Sector, Biotechnology Research Institute, National Research Council (Canada), 6100, Royalmount Avenue, Montreal, Quebec, Canada H4P 2R2, Department of Biochemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2H7, and Department of Biochemistry and Molecular Genetics, University of Colorado Health Sciences Center, 4200 E. 9th Avenue, Denver, Colorado 80262