1. Protein Crystallography Section, Solid State Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400085, India, National Institute for Research in Reproductive Health, Parel, Mumbai-400074, India, and LCCP/GSY, Institute de Biologie Structurale, J.-P. Ebel CEA-CNRS-UJF, 41, rue Jules Horowitz, F-38027 Grenoble Cedex 1, France