Sealy Center for Structural Biology, Department of Human Biological Chemistry and Genetics, University of Texas Medical Branch, Galveston TX 77555-1157, USA. cathy@newton.utmb.edu|Sealy Center for Structural Biology, Department of Human Biological Chemistry and Genetics, University of Texas Medical Branch, Galveston TX 77555-1157, USA. cathy@newton.utmb.edu|Sealy Center for Structural Biology, Department of Human Biological Chemistry and Genetics, University of Texas Medical Branch, Galveston TX 77555-1157, USA. cathy@newton.utmb.edu|Sealy Center for Structural Biology, Department of Human Biological Chemistry and Genetics, University of Texas Medical Branch, Galveston TX 77555-1157, USA. cathy@newton.utmb.edu