Molecular Biology Institute, UCLA-DOE Laboratory of Structural Biology & Molecular Medicine, University of California at Los Angeles, PO Box 951570, Los Angeles, CA 90095-1570, USA.|Molecular Biology Institute, UCLA-DOE Laboratory of Structural Biology & Molecular Medicine, University of California at Los Angeles, PO Box 951570, Los Angeles, CA 90095-1570, USA.|Molecular Biology Institute, UCLA-DOE Laboratory of Structural Biology & Molecular Medicine, University of California at Los Angeles, PO Box 951570, Los Angeles, CA 90095-1570, USA.