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Protein Has mRNA-like Function, Recruits tRNAs During Stalled Protein Translation

NEW YORK (GenomeWeb) – Scientists have found a protein that can induce the assembly of amino acids without DNA and the intermediary messenger RNA ordained by the textbook account of protein synthesis, according to a study published today in Science.

The researchers, led by Peter Shen, a postdoc at the University of Utah, observed a case in which one protein specifies which amino acids are added to another protein — a never-before-seen function for the molecules.

"We have a protein playing a role normally filled by mRNA," study co-author Adam Frost, an assistant professor at the University of California, San Francisco, said in a statement.

The scientists said they discovered the phenomenon using a technique called cryo-electron microscopy to flash freeze, and then visualize ribosomes that had stalled while assembling proteins. They were able to image the protein Rqc2, a subunit of the ribosome quality-control apparatus, as it attached random chains of the amino acids alanine and threonine to the protein being assembled. "We caught Rqc2 in the act," Frost said. "But the idea was so far-fetched."

To prove the unorthodox process was real, the scientists used RNA sequencing techniques to show that the Rqc2/ribosome complex had the potential to add amino acids to stalled proteins. The Rqc2 protein recruits tRNAs, molecules that bring amino acids to the protein assembly complex, specifically only tRNAs that carry alanine and threonine. The scientists were also able to show that the stalled proteins had extensive chains of alanines and threonines added to them.

The unusual sequence of amino acids could signal that the partial protein must be destroyed, or it could be part of a check to see whether the ribosome is working properly.

"Nature is capable of more than we realize," Shen, the lead author, said.