Protein That Can Edit Other Proteins Without

DNA Blueprint Discovered January 3, 2015 | by Janet Fang

photo credit: The Rqc2 protein (yellow) binds tRNAs (dark blue, teal), which add amino acids

(bright spot in middle) to a partially-made protein (green). The complex binds the ribosome

(white) / Janet Iwasa, Ph.D., University of Utah

In our cells, proteins are the tiny machines that do most of the work. And the instructions for

making proteins -- and for piecing together their building blocks, called amino acids -- are laid

out by DNA, then relayed through RNA. But now, researchers show for the first time that amino

acids can be assembled by another protein -- without genetic instructions. These

surprising findings were published in Science this week.

If a cell is an automobile-making factory, then ribosomes are the machines on the protein

assembly line that links together amino acids in an order specified by DNA and messenger

RNA (mRNA), an intermediate template. If something goes awry and a ribosome stalls, the

quality control team shows up to disassemble the ribosome, discard that bit of genetic

blueprint, and recycle the partially-made protein.

Turns out, that assembly line can keep going even if it loses its genetic instructions, according

to a large U.S. team led by University of Utah, University of California, San Francisco, and

Stanford researchers. They discovered an unexpected mechanism of protein synthesis where

a protein, and not the normal genetic blueprint, specifies which amino acids are added.

"In this case, we have a protein playing a role normally filled by mRNA," UCSFs Adam

Frost says in a news release. "I love this story because it blurs the lines of what we thought

proteins could do."

Frost and colleagues found a never-before-seen role for one member of the quality control

team: a protein named Rqc2, which helps recruit transfer RNA (tRNA) to sites of ribosomal

breakdowns (tRNA is responsible for bringing amino acids to the protein assembly line). Before

the incomplete protein gets recycled, Rqc2 prompts the stalled ribosomes to add two amino

acids -- alanine and threonine -- over and over. And that's because the Rqc2ribosome

complex binds tRNAs that carry those two specific amino acids. In the auto analogy, the

assembly line keeps going despite having lost its instructions, picking up whatever it can and

attaching it in no particular order: horn-wheel-wheel-horn-wheel-wheel-wheel-wheel-horn, for

example.

Pictured above, Rqc2 (yellow) binds tRNAs (blue and teal), which add amino acids (bright sot

in the middle) to a partially-made protein (green). The complex binds the ribosome (white). A

truncated protein with a seemingly random sequence of alanines and threonines probably

doesn't work properly, and that tail could be a code that signals for the malformed protein to be

destroyed.

http://www.iflscience.com/health-and-medicine/protein-directs-protein-synthesis-without-dna-

blueprint