Department of Biochemistry, National University of Singapore, Singapore. bernett@bic.nus.edu.sg <bernett@bic.nus.edu.sg>|Department of Biochemistry, National University of Singapore, Singapore. bernett@bic.nus.edu.sg <bernett@bic.nus.edu.sg>|Department of Biochemistry, National University of Singapore, Singapore. bernett@bic.nus.edu.sg <bernett@bic.nus.edu.sg>