1. Department of Biochemistry and School of Chemistry, University of Hyderabad, Hyderabad 500 046, India, Laboratory of Molecular and Cellular Biology, CDFD, Nacharam, Hyderabad 500 076, India, National Institute of Immunology, New Delhi 110 067, India, JNCASR, Bangalore, India, and Department of Biological Chemistry, Howard Hughes Medical Institute, University of Michigan Medical Center, Ann Arbor, Michigan 48109