Dominant Intermediate Charcot-Marie-Tooth disorder is not due to a catalytic defect in tyrosyl-tRNA synthetase

Charcot-Marie-Tooth disorder (CMT) is the most common inherited peripheral neuropathy, afflicting 1 in every 2500 Americans. One form of this disease, Dominant Intermediate Charcot-Marie-Tooth disorder type C (DI-CMTC), is due to mutation of the gene encoding the cytoplasmic tyrosyl-tRNA synthetase (TyrRS). Three different TyrRS variants have been found to give rise to DI-CMTC: replacing glycine at position 41 by arginine (G41R), replacing glutamic acid at position 196 by lysine (E196K), and deleting amino acids 153-156 (Δ(153-156)). To test the hypothesis that DI-CMTC is due to a defect in the ability of tyrosyl-tRNA synthetase to catalyze the aminoacylation of tRNA(Tyr), we have expressed each of these variants as recombinant proteins and used single turnover kinetics to characterize their abilities to catalyze the activation of tyrosine and its subsequent transfer to the 3' end of tRNA(Tyr). Two of the variants, G41R and Δ(153-156), display a substantial decrease in their ability to bind tyrosine (>100-fold). In contrast, the E196K substitution does not significantly affect the kinetics for formation of the tyrosyl-adenylate intermediate and actually increases the rate at which the tyrosyl moiety is transferred to tRNA(Tyr). The observation that the E196K substitution does not decrease the rate of catalysis indicates that DI-CMTC is not due to a catalytic defect in tyrosyl-tRNA synthetase.