1. M. B. Mixon, D. E. Coleman, and S. R. Sprang are in the Department of Biochemistry and E. Lee and A. G. Gilman are in the Department of Pharmacology, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75235-9050, USA.
2. A. M. Berghuis and S. R. Sprang are also with the Howard Hughes Medical Institute at the University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75235-9050, USA.