Domain movements during CCA-addition: A new function for motif C in the catalytic core of the human tRNA nucleotidyltransferases

Here, we show that motif C, whose actual function has not been identified yet, contributes to the switch in nucleotide specificity during polymerization. Biochemical characterization as well as EPR spectroscopy measurements of the human enzyme reveal that mutating the highly conserved amino acid position D139 in this motif interferes with AMP incorporation and affects interdomain movements in the enzyme. We propose a model of action, where motif C forms a flexible spring element modulating the relative orientation of the enzyme's head and body domains to accommodate the growing 3′-end of the tRNA.

Furthermore, these conformational transitions initiate the rearranging of the templating amino acids to switch the specificity of the nucleotide binding pocket from CTP to ATP during CCA-synthesis.