Scientists have found for the first time that the building blocks of a protein can be assembled by another protein and without genetic instructions. The findings defy textbook science which says DNA spells out the instructions for making proteins, tiny machines that do much of the work in our body's cells.
Scientists have now shown for the first time that the building blocks of a protein, called amino acids, can b assembled without blueprints - DNA and an intermediate template called messenger RNA (mRNA). Researchers observed a case in which another protein specifies which amino acids are added.
"This surprising discovery reflects how incomplete our understanding of biology is," said first author Peter Shen, a postdoctoral fellow in biochemistry at the University of Utah.
To put the new finding into perspective, it might help to think of the cell as a well-run factory, researchers said. Ribosomes are machines on a protein assembly line, linking together amino acids in an order specified by the genetic code. When something goes wrong, the ribosome can stall, and a quality control crew is summoned to the site. To clean up the mess, the ribosome is disassembled, the blueprint is discarded, and the partly made protein is recycled, researchers said. The study found a surprising role for one member of the quality control team, a protein named Rqc2.
Before the incomplete protein is recycled, Rqc2 prompts the ribosomes to add just two amino acids (of 20 total) - alanine and threonine - over and over, and in any order.
"In this case, we have a protein, Rqc2, playing a role similar to that of mRNA," said Adam Frost, assistant professor at University of California, San Francisco (UCSF) and adjunct professor of biochemistry at the University of Utah.
A truncated protein with an apparently random sequence of alanines and threonines looks strange, and probably doesn't work normally. But the nonsensical sequence likely serves specific purposes, researchers said. The code could signal that the partial protein must be destroyed, or it could be part of a test to see whether the ribosome is working properly. Evidence suggests that either or both of these processes could be faulty in neurodegenerative diseases such as Alzheimer's, Amyotrophic lateral sclerosis (ALS), or Huntington's.
Researchers used new RNA sequencing techniques to show that the Rqc2/ribosome complex had the potential to add amino acids to stalled proteins because it also bound transfer RNAs (tRNAs), structures that bring amino acids to the protein assembly line. The specific tRNAs researchers saw only carry the amino acids alanine and threonine. The study is published in the journal Science. PTI
