1. Departments of Biomolecular Discovery and Analytical Sciences, SmithKline Beecham Pharmaceuticals, R & D, King of Prussia, Pennsylvania 19406-0939, and Biochemistry & Cellular Biology Group, SmithKline Beecham Pharmaceutiques, Unit'e de Recherche, 4 rue du Chesnay Beauregard, BP 58, 35762 Saint Gregoire, France