[1] Queen Mary Univ London, William Harvey Res Inst, Barts & London Sch Med & Dent, London EC1M 6BQ, England
[2] Univ Amsterdam, Acad Med Ctr, Dept Mol Cell Biol, Sanquin Res & Landsteiner Lab, NL-1066 CX Amsterdam, Netherlands
[3] Max Planck Inst Biochem, Dept Mol Med, D-82152 Martinsried, Germany