1. Program in Structural Biology and Laboratory of Molecular Immunoregulation, National Cancer Institute, Frederick, Maryland 21702, USA, Rega Institute for Medical Research, Laboratory of Molecular Immunology, University of Leuven, Minderbroedersstraat 10, B-3000 Leuven, Belgium, and Technical University of Lodz, Institute of Technical Biochemistry, Stefanowskiego 4/10, 90-924 Lodz, Poland