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The Skaggs Institute
for Chemical Biology


Scientific Report 2007




Components of the Translation Apparatus in Neurologic Disease


P. Schimmel, J. Bacher, K. Beebe, R. Belani, Y. Chong, Z. Druzina, P. Fanta, M. Guo, M. Hanan, I.L. Jung, M. Kapoor, S. Kim, S.H. Lee, J. Liu, E. Merriman, M. Mock, C. Motta, M.H. Nawaz, F. Otero, Y.Z. Song, M.-N. Vo, W.F. Waas, W. Xie, X.-L. Yang, W. Zhang, Q. Zhou

Charcot-Marie-Tooth (CMT) disease is the most common heritable peripheral neuropathy in humans. Among other problems, patients with this disease have muscular weakness and atrophy in their distal extremities, no or diminished tendon reflexes, and impaired sensation. The ataxia (difficulty in walking) the patients experience is caused by defective neuronal conduction in axons. Remarkably, a subgroup of patients who have CMT disease have mutations in genes for either of 2 aminoacyl tRNA synthetases. Members of the tRNA synthetase family of enzymes are components of the translation apparatus found in all living organisms. The translation apparatus converts, or translates, the information in genes (DNA) into proteins.

GARS and YARS, genes for glycyl-tRNA synthetase (GlyRS) and tyrosyl-tRNA synthetase (TyrRS), respectively, are among the different genetic loci causally linked to CMT disease. The disease-causing mutations are dominant. At least 9 distinct mutant alleles in humans have been reported for GARS. Several of these genes are fully active in their functions for translation. These mutations cause CMT2D, a subtype of CMT disease characterized by a slowly progressing neuropathy that affects mostly the distal extremities. In a model developed by our collaborators at Jackson Laboratories, Bar Harbor, Maine, mice have a gene that encodes a mutant GlyRS that is fully active for aminoacylation. The mutant mice have the CMT2D phenotype of reduced nerve conduction velocities, a loss of axons with large diameters, and no defects in myelination. Because a deficit of aminoacylation function per se is not the cause of CMT, mice with a loss-of-function (aminoacylation) allele created by a gene-trap insertion were normal.

We have determined the x-ray crystal structures of the homodimeric human GlyRS and TyrRS molecules. Placing the disease-causing mutations onto the structure of human GlyRS showed them within a band encompassing both sides of the dimer interface. Strikingly,
2 CMT-causing mutations are complementary partners of a kissing contact across the dimer interface. Further analyses indicated that most mutations affect dimer formation (to enhance or weaken). A subset of 7 mutant proteins and the wild-type protein were expressed in transfected neuroblastoma cells that sprout primitive neurites. Although the wild-type protein was distributed into the nascent neurites and was associated with normal sprouting, all mutant proteins had defective distribution. Thus, the CMT-causing mutations of GlyRS have a common defect that may be connected in some way to a change in surfaces at the dimer interface.

The undisturbed ability of several of the mutant proteins to function in translation suggests a novel, new function of tRNA synthetases in neurogenesis. Expanded functions for tRNA synthetases in human cells have been demonstrated in other instances. For example, tryptophanyl-tRNA synthetase is converted to a cytokine that is a potent antiangiogenesis factor. Our future goal is to understand the functions of GlyRS and TyrRS in neurogenesis. Currently, we are identifying potential interacting partners of GlyRS and TyrRS in neuronal cells. The activities of the synthetases in neurogenesis likely are manifested through the complexes they form with these partners.

Publications

Bacher, J, Schimmel, P. An editing-defective tRNA synthetase is mutagenic in aging bacteria via the SOS response. Proc. Natl. Acad. Sci. U. S. A. 104:1907, 2007.

Bacher, J., Waas, W.F., Metzgar, D., de Crécy-Lagard, V., Schimmel, P. Genetic code ambiguity confers a selective advantage on Acinetobacter baylyi. J. Bacteriol. 189:6494, 2007.

Beebe, K., Waas, W., Druzina, Z., Guo, M., Schimmel, P. A universal plate format for increased throughput of assays that monitor multiple aminoacyl tRNA synthetase activities. Anal. Biochem. 368:111, 2007.

Nangle, L., Zhang, W., Xie, W., Yang, X.-L., Schimmel, P. Charcot-Marie-Tooth disease-associated mutant tRNA synthetases linked to altered dimer interface and neurite distribution defect. Proc. Natl. Acad. Sci. U. S. A. 104:11239, 2007.

Waas, W.F., Druzina, Z., Hanan, M., Schimmel, P. Role of a tRNA base modification and its precursors in frameshifting in eukaryotes. J. Biol. Chem. 282:26026, 2007.

Waas, W.F., Schimmel, P. Evidence that tRNA synthetase-directed proton transfer stops mistranslation. Biochemistry 46:12062, 2007.

Xie, W., Nangle, L.A., Zhang, W., Schimmel, P., Yang, X.-L. Long-range structural effects of a Charcot-Marie-Tooth disease-causing mutation in human glycyl-tRNA synthetase. Proc. Natl. Acad. Sci. U. S. A. 104:9976, 2007.

Yang, X.-L., Guo, M., Kapoor, M., Ewalt, K.L., Otero, F.J., Skene, R.J., McRee, D.E., Schimmel, P. Functional and crystal structure analysis of active site adaptations of a potent anti-angiogenic human tRNA synthetase. Structure 15:793, 2007.

 

Paul R. Schimmel, Ph.D.
Ernest and Jean Hahn Professor and Chair of Molecular Biology and Chemistry

Schimmel Web Site