In vitro studies of disease-linked variants of human tRNA nucleotidyltransferase reveal decreased thermal stability and altered catalytic activity.

Mutations in the human TRNT1 gene encoding tRNA nucleotidyltransferase (tRNA-NT), an essential enzyme responsible for addition of the CCA (cytidine-cytidine-adenosine) sequence to the 3'-termini of tRNAs, have been linked to disease phenotypes including congenital sideroblastic anemia with B-cell immunodeficiency, periodic fevers and developmental delay (SIFD) or retinitis pigmentosa with erythrocyte microcytosis. The effects of these disease-linked mutations on the structure and function of tRNA-NT have not been explored. Here we use biochemical and biophysical approaches to study how five SIFD-linked amino acid substitutions (T154I, M158V, L166S, R190I and I223T), residing in the N-terminal head and neck domains of the enzyme, affect the structure and activity of human tRNA-NT in vitro. Our data suggest that the SIFD phenotype is linked to poor stability of the T154I and L166S variant proteins, and to a combination of reduced stability and altered catalytic efficiency in the M158 V, R190I and I223T variants.

The Saccharomyces cerevisiae enolase-related regions encode proteins that are active enolases.

In addition to two genes (ENO1 and ENO2) known to code for enolase (EC4.2.1.11), the Saccharomyces cerevisiae genome contains three enolase-related regions (ERR1, ERR2 and ERR3) which could potentially encode proteins with enolase function. Here, we show that products of these genes (Err2p and Err3p) have secondary and quaternary structures similar to those of yeast enolase (Eno1p). In addition, Err2p and Err3p can convert 2-phosphoglycerate to phosphoenolpyruvate, with kinetic parameters similar to those of Eno1p, suggesting that these proteins could function as enolases in vivo. To address this possibility, we overexpressed the ERR2 and ERR3 genes individually in a double-null yeast strain lacking ENO1 and ENO2, and showed that either ERR2 or ERR3 could complement the growth defect in this strain when cells are grown in medium with glucose as the carbon source. Taken together, these data suggest that the ERR genes in Saccharomyces cerevisiae encode a protein that could function in glycolysis as enolase. The presence of these enolase-related regions in Saccharomyces cerevisiae and their absence in other related yeasts suggests that these genes may play some unique role in Saccharomyces cerevisiae. Further experiments will be required to determine whether these functions are related to glycolysis or other cellular processes.

Analysis of the pathogenic I326T variant of human tRNA nucleotidyltransferase reveals reduced catalytic activity and thermal stability in vitro linked to a conformational change.

The I326T mutation in the TRNT1 gene encoding human tRNA nucleotidyltransferase (tRNA-NT) is linked to a relatively mild form of SIFD. Previous work indicated that the I326T variant was unable to incorporate AMP into tRNAs in vitro, however, expression of the mutant allele from a strong heterologous promoter supported in vivo CCA addition to both cytosolic and mitochondrial tRNAs in a yeast strain lacking tRNA-NT. To address this discrepancy, we determined the biochemical and biophysical characteristics of the I326T variant enzyme and the related variant, I326A. Our in vitro analysis revealed that the I326T substitution decreases the thermal stability of the enzyme and causes a ten-fold reduction in enzyme activity. We propose that the structural changes in the I326T variant that lead to these altered parameters result from a rearrangement of helices within the body domain of the protein which can be probed by the inability of the monomeric enzyme to form a covalent dimer in vitro mediated by C373. In addition, we confirm that the effects of the I326T or I326A substitutions are relatively mild in vivo by demonstrating that the mutant alleles support both mitochondrial and cytosolic CCA-addition in yeast.