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Advance Online Publication|doi:10.1038/nature05002|Published online 13 August 2006
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MOLECULAR BIOLOGY
Sticky end in protein synthesis
Hervé Roy and Michael Ibba
It’s not clear what general level of accuracy is required in translating the genetic code. But the protective
role of proof-reading is evident from a case in which a small mistake has a catastrophic effect.
When protein production in a cell goes awry,
abnormal deposits can form and contribute to
various diseases known collectively as amyloidoses1. The tendency of certain proteins to
aggregate can be increased by mutations in the
genes that encode them, as in Huntington’s or
Alzheimer’s diseases. Or aggregation can be
induced by an external factor such as a prion,
for example in conditions such as mad cow
disease or kuru.
In a paper published on Nature’s website
today, Lee et al.2 describe another pathway
by which pathological cellular aggregates can
form. Certain mutant mice, called ‘sticky’ mice
because of the effects on their fur, also suffer
from neurodegeneration caused by the death
of a type of brain cell — the Purkinje cell —
induced by protein aggregation. In investigating the origins of this neuron death, Lee et al.
unexpectedly identified a mutation in the gene
coding for alanyl-tRNA synthetase (AlaRS), a
component of the gene-expression machinery
by which aminoacyl-transfer RNAs translate
messenger RNA into an amino-acid sequence
on the ribosome. Closer inspection revealed
that this mutation increases the frequency of
errors during translation, leading to the gradual accumulation of inaccurately synthesized
proteins that eventually form aggregates. As
well as discovering another cause of cellular
protein aggregation, Lee et al. demonstrate the
importance of quality control by the aminoacyl-tRNA synthetases (aaRSs), the family to
which AlaRS belongs, in the intricate context
of a multicellular organism.
The fidelity of information transfer is
carefully controlled at each step during gene
expression (Fig. 1). The most error-prone steps
occur at the ribosome, where an aminoacyltRNA is matched with the corresponding
codon (specifying an amino acid) on the messenger RNA, and an amino acid is paired with
a tRNA by an aaRS3. With a few exceptions4,
mistakes from the aminoacylation reaction
cannot be corrected, which almost inevitably
leads to the incorporation of the wrong amino
acid during protein synthesis5. AlaRS is the
aaRS that attaches the amino acid alanine (Ala)
to its corresponding transfer RNA (tRNAAla)
during protein synthesis.
DNA replication
Error rate 10–8–10–10
+
Amino
acid
Transcription into mRNA
10–4
tRNA
aaRS
Ribosome
Translation of mRNA
10–4
Aminoacylation of tRNA
10–3–10–4
Selection of tRNAs by ribosomes
10–3–10–5
Maturation
Misfolded
protein
Aggregation
Functional
protein
Figure 1 | Accuracy in gene expression. Transfer of information from the DNA sequence of a gene to the
corresponding amino-acid sequence of a protein requires transcription of the sequence into messenger
RNA, then translation of that RNA into an amino-acid sequence at ribosomes, through the agency of
amino-acid-specific transfer RNAs. These steps are all prone to error (typical rates are shown in red). The
inset shows the aminoacylation of tRNA, in which an amino acid is paired with a tRNA by the enzyme
aminoacyl-tRNA synthetase (aaRS). It is a failure of quality control at this stage that Lee et al.2 show
causes neurodegeneration in ‘sticky mice’. Once synthesized, a protein usually becomes functional.
In some cases, however, misfolding occurs. Most misfolded proteins are degraded by the cell — but
some form insoluble aggregates that, as in the sticky mouse, can lead to disease.
The mutation Lee et al. identified does not
impair the expression, structure or solubility
of AlaRS, nor does it reduce the capacity for
Ala-tRNAAla synthesis; instead, the mutated protein is less able to recognize and correct its own
mistakes, leading to the accidental attachment
© 2006 Nature Publishing Group
of serine to tRNAAla. This is because the mutation leads to a sequence impairment in the
AlaRS ‘editing site’, the domain of the protein
that normally ensures that incorrectly activated
amino acids are removed before they can be
used for protein synthesis6. Editing by aaRSs is
1
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found in numerous systems, and serves to
eliminate errors that inevitably arise when trying to discriminate between pairs of similar
amino acids7.
In the case of AlaRS, serine is mistakenly
activated and attached to tRNA at the active site
with a frequency of about 1 in 500 compared
with alanine. Then, rather than being released
for protein synthesis, as occurs for alanine,
the misactivated serine moves to the editing
site where it is removed from the tRNA. It is
this final step that is specifically impaired in
the sticky mouse; Lee and colleagues’ in vitro
analyses2 showed that, rather than hydrolysing
Ser-tRNAAla, the mutated AlaRS produces more
of this mischarged species, leading to errors in
protein synthesis when alanine codons are mistakenly translated as serine. The results of these
mistakes, the frequency of which is unknown,
are protein misfolding, accumulation of protein
aggregates and progressive neurodegeneration.
Editing by aaRSs has long been assumed
to be a point of quality control in translation,
although evidence for this role has been sparse
until now. Studies in microorganisms showed
that defects in editing impaired competitive-
2
NATURE|AOP|doi:10.1038/nature05002|Published online 13 August 2006
ness but were not usually lethal8,9, suggesting that cells can cope with a certain degree
of error at this final step of gene expression.
The work of Lee et al.2 shows just how fine the
line is between a tolerable degree of error and
a catastrophic loss of accuracy in protein synthesis. The loss in editing activity caused by the
mutated AlaRS has no discernible effect on the
efficiency of protein synthesis in non-neuronal
cells, but has a detrimental effect on Purkinje
cells. This may be because, as these cells do not
divide, there is no ‘dilution’ of the misfolded
proteins by cell division. One question not yet
addressed is whether the sticky-mouse characteristics arise from elevated serine misincorporation during synthesis of a particular subset of
proteins in Purkinje cells, or from a general loss
of fidelity in translation.
The discovery that ‘upstream’ defects in protein synthesis might lead to amyloidoses could
open new routes for treating these devastating
diseases. For example, just as Lee et al. show
that in mutant mouse cells elevated serine
levels decrease viability, one can naively speculate that a serine-restricted diet might limit
erroneous protein synthesis to a level that
© 2006 Nature Publishing Group
neuronal cells can tolerate. Clearly, this is an
oversimplification, and much remains to be
learned both about the formation of cellular
protein aggregates and how failures in quality
control contribute to this and perhaps other
diseases. However that turns out, it is clear that
the margin for error in translation is smaller
than we thought.
■
Hervé Roy and Michael Ibba are in the
Department of Microbiology and the Ohio
State Biochemistry Program, Ohio State
University, Columbus, Ohio 43210-1292, USA.
e-mail: [email protected]
1. Merlini, G. & Bellotti, V. N. Engl. J. Med. 349, 583–596
(2003).
2. Lee, J. W. et al. Nature doi:10.1038/nature05096 (2006).
3. Ibba, M. & Söll, D. Science 286, 1893–1897 (1999).
4. Dale, T. & Uhlenbeck, O. C. Trends Biochem. Sci. 30,
659–665 (2005).
5. Wang, L., Xie, J. & Schultz, P. G. Annu. Rev. Biophys. Biomol.
Struct. 35, 225–249 (2006).
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22, 668–675 (2003).
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(1992).
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& Schimmel, P. J. Biol. Chem. 277, 45729–45733 (2002).
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