[1] Harvard Univ, Sch Med, Dept Biol Chem & Mol Pharmacol, Boston, MA 02115 USA
[2] Boehringer Ingelheim Canada Ltee, Biomega Div Rech, Laval, PQ H7S 2G5, Canada
[3] Univ York, Dept Chem, York YO10 5DD, N Yorkshire, England
[4] Univ Connecticut, Dept Mol & Cell Biol, Storrs, CT 06269 USA