[1] Natl Inst Med Res, MRC, London NW7 1AA, England
[2] Univ London Imperial Coll Sci Technol & Med, Div Mol Biosci, London, England
[3] Inst Res Biomed, CH-6500 Bellinzona, Switzerland
[4] Univ Cambridge, Dept Chem, Cambridge CB2 1EW, England
[5] Kings Coll London, Dept Basic & Clin Neurosci, London WC2R 2LS, England