1. Department of Biochemistry, University of Hyderabad, Hyderabad 500 046, Andhra Pradesh, India, National Institute of Immunology, Aruna Asaf Ali Marg, New-Delhi 100 067, India, Molecular and Cellular Biology Laboratory, CDFD, Nacharam, Hyderabad 500 076, India, and Department of Biological Chemistry, Howard Hughes Medical Institute, University of Michigan Medical Center, Ann Arbor, Michigan 48109