1. Henry Wellcome Laboratories for Integrated Cell Signalling and Department of Biochemistry, School of Medical Sciences, University of Bristol, U.K. The current affiliation for S.J.B. is the Department of Pharmacy and Pharmacology, University of Bath, Claverton Down, Bath, U.K. The current affiliation for G.A.R. is the Department of Cell Biology, Division of Medicine, Faculty of Medicine, Imperial