The Skaggs Institute
for Chemical Biology
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,
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.
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.
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.