[1] Harvard Univ, Sch Med, Dept Biol Chem & Mol Pharmacol, Boston, MA 02115 USA
[2] Harvard Univ, Sch Med, Dana Farber Canc Inst, Boston, MA 02115 USA
[3] Kings Coll London, Dept Chem, London SE1 1UL, England
[4] Kings Coll London, Randall Div Cell & Mol Biophys, London SE1 1UL, England
[5] Kings Coll London, Dept Phys, London WC2R 2LS, England
[6] Kings Coll London, Randall Div Cell & Mol Biophys, London WC2R 2LS, England
[7] Univ Munster, Inst Neurobiol, D-48149 Munster, Germany
[8] Harvard Univ, Sch Med, Dept Syst Biol, Boston, MA 02115 USA