Laboratory of Structural Biology, MR-01, National Institute of Environmental Health Sciences, National Institutes of Health, Box 12233, Research Triangle Park, NC 27709, USA.|Laboratory of Structural Biology, MR-01, National Institute of Environmental Health Sciences, National Institutes of Health, Box 12233, Research Triangle Park, NC 27709, USA.|Laboratory of Structural Biology, MR-01, National Institute of Environmental Health Sciences, National Institutes of Health, Box 12233, Research Triangle Park, NC 27709, USA.|Laboratory of Structural Biology, MR-01, National Institute of Environmental Health Sciences, National Institutes of Health, Box 12233, Research Triangle Park, NC 27709, USA.