1Georgia Institute of Technology, School of Biology, School of Chemistry and Biochemistry, 310 Ferst Drive, Rm. A118, Atlanta, GA 30332-0230, USA
2Molecular Structure and Function Program, The Hospital for Sick Children, and Departments of Biochemistry and Medical Biophysics, The University of Toronto, 555 University Avenue, Rm. 3330, Toronto, ON, Canada, M5G 1X8