Making proteins -- the most important activity within any cell -- was always thought to require a "blueprint" provided by DNA, but researchers say they've discovered some proteins that can make other proteins without needing any such instructions.
The usual protein "blueprint," provided by DNA and an intermediate pattern called messenger RNA (mRNA), causes proteins to be assembled from amino acids within a cellular structure known as a ribosome.
However, researchers are reporting in the journal Science a discovery of some cases in which a kind of protein can "edit" other proteins by specifying which exact amino acids should be added.
"This surprising discovery reflects how incomplete our understanding of biology is," says study first author Peter Shen, a biochemistry postdoctoral fellow at the University of Utah. "Nature is capable of more than we realize."
The protein, Rqc2, has been found to come into play when the normal process of amino acids being assembled following genetic instructions goes wrong.
Normally when that happens the ribosome comes apart as the blueprint is discarded and the partially built protein gets recycled.
However, the protein Rqc2 has been found capable of prompting the ribosome to stay intact and add just two amino acids, alanine and threonine, again and again in any order.
As part of the recycling process, this protein-editing behavior of Rqc2 may be a significant part of keeping the body clear of complete but faulty proteins, the researchers suggest.
It's possible the string of amino acids added following the Rqc2 instructions marks the protein for elimination, or it could be a method of testing whether a ribosome is working correctly, they suggest.
"In this case, we have a protein playing a role normally filled by mRNA," says researcher Adam Frost, an adjunct professor of biochemistry at the University of Utah. "I love this story because it blurs the lines of what we thought proteins could do."
The scientists were drawn to the odd behavior of Rqc2 when they utilized a technique known as cryo-electron microscopy to freeze and visualize the quality control process within cells, of which the protein is a part, in action.
"We caught Rqc2 in the act," says Frost. "But the idea was so far-fetched. The onus was on us to prove it."
Faulty quality control processes in the production of proteins may be behind certain human disorders such as Alzheimer's or Huntington's diseases, the researchers say, so understanding the conditions under which Rqc2 is triggered, and when or why it fails to trigger, could yield important advances in developing new treatments for such neurodegenerative diseases.