1. Contribution from the Laboratory for Bioorganic Chemistry, Sloan-Kettering Institute for Cancer Research, 1275 York Avenue, New York, New York 10021; Program in Cell Biology and Department of Medicine, Sloan-Kettering Institute for Cancer Research, 1275 York Avenue, New York, New York 10021; and Department of Chemistry, Columbia University, Havemeyer Hall, 3000 Broadway, New York, New York 10027