Protein synthesis, the processes whereby DNA is transcribed and translated into functional proteins, is the most essential function of the cell, “practically gospel in the field of molecular biology.”
Recent research, however, has identified exceptions to the gospel. Normally, genomic DNA is transcribed into messenger RNA (mRNA). After post-transcriptional modifications, this mRNA travels to ribosomes, which uses a related molecule (transfer RNA or tRNA) to translate the mRNA transcript into a sequence of amino acids. As the amino acids are made, they fold and twist into proteins, which can also undergo post-translational modifications to become fully functional.
In a study published today in the prestigious journal Science, the authors reveal cryo-electron microscopy images that illustrate an entirely different way of building a protein. In this case, the protein Rqc2 behaves like an mRNA transcript; it is directly connected to tRNA and directs this structure to insert amino acids into the string of proteins being created.
Apparently, the actions of Rqc2 are not a case of “a protein going rogue.” As described by Nick Stockton for Wired.com, the actions seem to instead “be part of the recycling process that occurs when there’s a mistake in a protein being built. When an error is introduced, the ribosomes stall and call in a group of quality control proteins, including Rqc2. In observing this process, the researchers saw how Rqc2 links up with the tRNA and tells it to insert a random sequence of two amino acids into the chain (out of 20 total amino acids.” This finding is significant; until recently, it was always believed that a DNA/RNA transcript is required to build a protein, but the insertion of these two amino acids demonstrates that this may not be true.
Researchers believe that this mechanism is an important component for the body to maintain error-free proteins. Though the exact function of these two amino acid insertions is unknown, scientists speculate that the insertion may flag the protein for destruction or the addition of the amino acids could be used to test if the ribosome is functional. Understanding the function of proteins such as Rqc2 may then shed light on a number of neurodegenerative diseases such as Alzheimer’s and amyotrophic lateral sclerosis (ALS), which have been shown to result in defective control processes.