1. University of Nottingham, Department of Applied Biochemistry & Food Science, Sutton Bonington LE12 5RD, U.K.
2. A.F.R.C. Laboratory of Molecular Signalling, Department of Zoology, University of Cambridge, Downing Street, CB2 3EJ, Cambridge, U.K.
3. University of Minnesota, Department of Biochemistry, St Paul, Minnesota, MN 55108, U.S.A.