Study reveals 2FA system that controls protein and microRNA destruction

Researchers have uncovered how cells selectively destroy certain microRNAs through protein degradation

To the Point

• Regulation by miRNA: MicroRNAs (miRNAs) help cells control which genes are active. They themselves must also be regulated—specifically through their degradation.

• Interaction of two signals: Cells selectively degrade certain miRNAs through protein degradation. miRNA degradation is triggered only when the Argonaut-miRNA complex and trigger RNA are present.

• Complex molecular interactions: The ZSWIM8-E3 ligase recognizes several structural changes that occur when the two RNAs—the miRNA and the trigger RNA—bind to each other within the Argonaut protein.

Cells rely on tiny molecules called microRNAs to tune which genes are active and when. Cells must carefully control the lifespan of microRNAs to prevent widespread disruption to gene regulation.

A new study led by researchers at Max Planck Institute of Biochemistry in Germany and at the American Whitehead Institute reveals how cells selectively eliminate certain microRNAs through an unexpectedly intricate molecular recognition system. The work shows that the process requires two separate RNA signals, similar to how many digital systems require two forms of identity verification before granting access. The findings explain how cells use this “two-factor authentication” system to ensure that only intended microRNAs are destroyed, leaving the rest of the gene regulation machinery in operation.

miRNAs destroy mRNA

Proteins are the molecular machines of a cell. To produce proteins, the genetic information in DNA is copied to messenger RNA, or mRNA. And then the ribosomes – the protein factories – read the mRNAs and produce proteins. One way to regulate this process is through microRNAs, or miRNA. MicroRNAs are short strands of RNA that help control gene expression. Working together with a protein called Argonaute, they bind to specific messenger RNAs and trigger their destruction. So that, microRNAs can reduce the production of specific proteins.

While scientists recognized that microRNAs could be destroyed through a pathway known as target-directed microRNA degradation, or TDMD, the details of how cells recognized which microRNAs to eliminate remained unclear.  “We knew there was a pathway that could target microRNAs for degradation, but the biochemical mechanism behind it wasn’t understood,” says David Bartel, Whitehead Institute Member and senior author of the study.

Earlier work from Bartel’s lab and others had identified a key player in this pathway: the ZSWIM8 E3 ubiquitin ligase. E3 ubiquitin ligases are involved in the cell’s recycling system and attach a small molecular tag called ubiquitin to other proteins, marking them for destruction.

The researchers first showed that the ZSWIM8 E3 ligase specifically binds and tags Argonaute, the protein that holds microRNAs and helps regulate genes. The researchers’ next challenge was to understand how this machinery recognized Argonaute complexes carrying only specific microRNAs that should be degraded.

Dual-RNA recognition process

The answer turned out to be surprisingly sophisticated. Using a combination of biochemistry and cryo-electron microscopy—an imaging technique that reveals molecular structures at near-atomic resolution—the researchers discovered that the degradation system relies on a dual-RNA recognition process. First, Argonaute must carry a specific microRNA. Second, another RNA molecule called a “trigger RNA” must bind to that microRNA in a particular way.

The degradation process is activated only when both signals—the Argonaute-miRNA complex and the trigger RNA—are present. This dual requirement ensures exquisite specificity. Each cell can contain over a hundred thousand Argonaute–microRNA complexes regulating many genes, and destroying them indiscriminately would disrupt essential biological processes.

The structure revealed complex molecular interactions. The ZSWIM8 ligase detects multiple structural changes that occur when the two RNAs bind together within the Argonaute protein. “When we saw the structure, everything clicked.” says Elena Slobodyanyuk, a graduate student in Bartel’s lab and co-first author of the study. “You could see how the pairing of the trigger RNA with the microRNA reshapes the Argonaute complex in a way that the ligase can recognize.”

Regulation of RNA

Beyond explaining how target-directed microRNA degradation works, the findings may impact how scientists think about the regulation of RNA molecules more broadly. “It was like opening a treasure chest where every detail revealed something new and mesmerizing.” says Jakob Farnung, co-first author and researcher in the Department of Molecular Machines and Signaling at the Max Planck Institute of Biochemistry. “ZSWIM8 recognizes not just the protein, and the RNA pair but also protein-RNA interfaces.”

“This opens up a whole new way of thinking about how RNA molecules can control protein degradation,” says Brenda Schulman study co-senior author and Director of the Department of Molecular Machines and Signaling at the Max Planck Institute of Biochemistry. “Here, the recognition was far more elaborate. There’s likely much more left to discover.”

Uncovering the details of this intricate regulatory system required interdisciplinary collaboration, combining expertise in RNA biochemistry, structural biology, and ubiquitin enzymology to solve this long-standing molecular puzzle.

 

Glossary:
Argonaute protein: often abbreviated as AGO, is a protein in the cell that works with miRNA. It helps identify specific mRNAs that contain instructions for proteins. Once the Argonaute-miRNA complex finds the target mRNA, it blocks its function—either by degrading it or by preventing it from being translated into a protein. The name “Argonaut” comes from Greek mythology—just as the Argonauts were searching for the Golden Fleece, the Argonaut protein “searches” for its target RNA.

mRNA: Abbreviation for messenger ribonucleic acid; it contains the genetic information for the structure of a protein. mRNA consists of four basic building blocks: the ribonucleotides adenine, guanine, cytosine, and uracil. The sequence of these ribonucleotides determines the sequence of amino acids, the basic building blocks of proteins.

MicroRNA (miRNA): a short, non-coding RNA that acts as a molecular regulator within the cell to silence genes by degrading messenger RNA or blocking its translation into proteins

TDMD: An abbreviation for target-directed microRNA degradation; this is a process in which a trigger RNA (e.g., an mRNA or a non-coding RNA) binds to the miRNA. This leads to the degradation of the miRNA. As a result, the miRNA’s function of inhibiting protein production is suppressed.

Trigger RNA: is an RNA molecule that destabilizes and destroys the miRNA by binding to miRNA-AGO complexes—a finely tuned mechanism for reversing gene silencing.

ZSWIM8 ligase: an abbreviation for "Zinc finger SWIM-type containing 8". Together with its protein partner cullin 3 and further ubiquitylating enzymes, ZSWIM8 marks proteins—particularly Argonaute proteins—with ubiquitin, thereby degrading AGO and subsequently miRNAs.