Even small mistakes made by cells during protein production can have profound disease effects, but the processes cells use to correct mistakes have been challenging to decipher.
Recent work by scientists at The Scripps Research Institute, however, has uncovered two surprising new methods for such editing.
The work, published in the January 3, 2008 issue of the journal Nature, and led by Professor Paul Schimmel of the Skaggs Institute for Chemical Biology at Scripps Research, could help identify underlying causes of a range of diseases and, in time, even ways to correct the errors.
Producing proteins is an essential but complicated process involving a number of components. Messenger RNA within cells act as the instructions for protein synthesis. Ribosomes are cellular structures that read these instructions and follow their directions to bind with bits of transfer RNA carrying the amino acids needed for a given protein chain. In most cases, a single unique form of transfer RNA binds only to a single amino acid, and a specific enzyme called a synthetase is responsible for joining the two.
In very rare cases—typically a fraction of a percent—transfer RNA binds with the wrong amino acid. If this error, or mistranslation, is not corrected, that mistranslated amino acid is ultimately incorporated into a protein. Past research by the Schimmel team and others has shown that as little as one wrong amino acid can have profound consequences for health.
"Even such a tiny defect can overwhelm a cell's ability to deal with misfolded proteins, ultimately causing specific neurological problems," says Scripps Research molecular biologist Kirk Beebe, first author of the new paper with Marissa Mock.
Quality Control
The prevailing thinking among researchers in the field has been that the portions of synthetases that recognize and bind transfer RNA and appropriate amino acids are so accurate that little editing is required. What little editing does occur was thought tied to the same checkpoints within the enzymes that perform recognition and binding.
But the current study suggests a new perspective is needed, at least for one widely studied enzyme, a synthetase found in everything from bacteria to humans that binds the amino acid alanine.
The Schimmel team's research revealed that a completely distinct segment of the enzyme acts as a second checkpoint responsible for identifying mistranslations and removing any amino acid besides alanine that might attach to the alanine transfer RNA. Remarkably, this second zone within the enzyme focuses its activity on the very same two nucleotides in the genetic code of the transfer RNA used by the first checkpoint, a guanine and uracil pair referred to as G3•U70.
"The part that is astonishing is that the information that each of these checkpoints is looking for is embedded in the same transfer RNA molecule," says Schimmel, "There's no precedent for this that we're aware of."
The researchers were able to show that this editor checkpoint, even when separated from the rest of the enzyme, was able to efficiently cleave mistranslated amino acids from the alanine transfer RNA. Further experiments revealed that when the G3•U70 pair was transferred to a different type of transfer RNA, the editing unit still removed a non-alanine amino acid, showing clearly that the pair is the trigger for the activity.
Mystery Solved?