One of the first things you learn in any science textbook, is that its our DNA which directs how proteins are made, by instructing the amino acids or the building blocks of proteins to be assembled in a certain order. This is also known as the central dogma of molecular biology. However, a recent study published in the journal Science, on Jan 2nd, shows for the first time that proteins can be assembled without instructions from DNA and intermediate template called messenger RNA(mRNA).
Protein synthesis can be compared to an assembly line process where, ribosomes are like machines adding and linking amino acids in an orderly fashion, as specified by the genetic code. When there is a mistake in this process, the ribosome stalls and a ribosome quality control complex (RQC) mediates the degradation of the incompletely synthesized, defective protein and recycles it. The team has found interesting insights about how a protein member of the RQC complex called Rqc2, which has been conserved from yeast to man, acts during the process. In the event of a mistake, the protein Rqc2 prompts the ribosome to add two amino acids, namely alanine and threonine randomly – over and over again, like an assembly line which has lost control. This results in a defective protein with a nonsensical sequence of alanine and threonine attached to it, which serves as a signal to the body to destroy it.
“In this case, we have a protein playing a role normally filled by mRNA,” says Adam Frost, M.D., Ph.D., assistant professor at University of California, San Francisco (UCSF) and adjunct professor of biochemistry at the University of Utah. The team used cryo-electron microscopy to flash freeze and visualize the quality control machinery in action. “We caught Rqc2 in the act,” says Frost. They have further done extensive biochemical analysis to prove their hypothesis.
“I love this story because it blurs the lines of what we thought proteins could do” says Dr. Jonathan Weissman, a Howard Hughes Medical Institute investigator at UCSF and a senior author of this paper. Indeed!
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