[1] Harvard Univ, Dept Cellular & Mol Biol, Cambridge, MA 02138 USA
[2] Childrens Hosp, Mol Med Lab, Boston, MA 02115 USA
[3] Harvard Univ, Howard Hughes Med Inst, Cambridge, MA 02138 USA
[4] Harvard Univ, Sch Med, Dept Cell Biol, Boston, MA 02115 USA
[5] Brandeis Univ, Dept Biochem, Waltham, MA 02454 USA
[6] EMBL, F-38000 Grenoble, France
[7] Natl Inst Med Res, London NW7 1AA, England
[8] Harvard Univ, Dept Chem & Biol Chem, Cambridge, MA 02138 USA