In the English society of former times a “chaperone”, traditionally an older woman, was assigned to accompany a young unmarried woman to ensure her proper behavior, especially during interactions with men, in line with the social norms of the time. In biochemistry, "chaperones" also play a protective role. One of their main functions is to assist newly synthesized proteins in folding correctly and to prevent misfolded protein chains from clumping. Other chaperones, known as “assembly chaperones,” help to bring together various building blocks in the densely packed cellular environment and arrange them into large protein complexes. Without these crucial functions, life as we know it would not be possible.

Now, scientists at the University of Würzburg have discovered a previously unknown type of assembly chaperone during their analysis of poxviruses, and they have decoded its function in full detail. The unique aspect: this is the first known chaperone that is not formed by a protein but by a nucleic acid — specifically RNA, even more precisely, a tRNA or "transfer RNA."

This study was led by a research team under Professor Utz Fischer, Chair of Biochemistry at the Julius Maximilian University of Würzburg (JMU) and an associate member of the Helmholtz Institute for RNA-based Infection Research (HIRI). Additional collaborators included Professors Claudia Höbartner and Bettina Warscheid from JMU’s Faculty of Chemistry and Pharmacy, as well as researchers from Innsbruck, Hanover, and Chicago. The team has now published the results of their work in the journal Nature Structural and Molecular Biology. These findings could serve as a basis for the development of new drugs against poxviruses.

“In our study, we focused on a large protein complex: the so-called complete vRNAP, an RNA polymerase found in vaccinia, the prototypical poxvirus” Fischer explains. This enzyme consists of 15 proteins and one RNA molecule and plays a crucial role in gene expression — the process by which genetic information is translated into proteins.

Each component of the complex has a specific task in this process. One factor helps the polymerase attach to the start sites (promoters) of viral genes, another enables it to detach from the promoters, and a third modifies the newly formed messenger RNA. “All in all, this protein complex acts like an ‘all-in-one unit,’” explains Dr. Julia Bartuli, who led the biochemical part of the study. What intrigued her most was the question of how so many proteins could be assembled into such a highly ordered structure. In other words: who is the architect of this complex? “To answer that, we combined biochemical and structural biology approaches to identify each individual step,” says the biochemist.

The team discovered that the complex is built by an assembly chaperone — a molecule that changes its structure while carrying out a specific task and then returns to its original form. To their surprise, they found that this chaperone is not made of protein but of RNA. “Typically, RNA has no role in this kind of process. Yet here a tRNA sits centrally between the polymerase and the associated factors, ensuring their cohesion and readiness to initiate gene expression,” explains Dr. Clemens Grimm, who was responsible for the structural analysis in the study. Further experiments on the role of tRNA revealed that without it, the other components of the complex have no affinity for each other and would not assemble correctly on their own. Only with the help of the tRNA do they come together in a specific sequence — during which the tRNA itself changes structure. This locks the system into place and stabilizes it.

The remaining question was: what is the purpose of this complex? To understand this, one needs in-depth knowledge of smallpox viruses: “Smallpox viruses are DNA viruses that never enter the nucleus of the infected cell. Instead, their replication occurs entirely in the cytoplasm. This means the virus must bring along everything it needs to initiate transcription and thus its own replication,” explains Utz Fischer. And that is precisely the role of complete vRNAP.

The complex is formed in a late stage of the infection and then integrated into a new virus particle. It functions there as a “kickstarter for transcription.” All essential components are bundled together to ensure a smooth start during the initial phase of infection. So essentially, this complex serves as a kind of “survival kit” that enables poxviruses to rapidly multiply within infected cells.

Although this is basic research on the vaccinia virus, the findings could have important implications given current developments in Africa. For the past three years, Mpox viruses have been emerging in several countries, causing localized outbreaks. Their relation to vaccinia is made clearer by their former name: until recently, they were known as "monkeypox."

Since Mpox viruses have so far only spread through close physical contact, the number of infections has remained relatively low — nothing like SARS-CoV-2, the virus responsible for the COVID-19 pandemic. However, this appears to be changing: “In Africa, we’re observing that the virus is mutating and finding new transmission routes,” says Utz Fischer.

For centuries, classical smallpox — caused by the poxvirus variola— ranked among the most dangerous diseases. Fischer even calls it the ultimate “killer.” The development of vaccinia-based vaccines and worldwide vaccination campaigns eventually led to its eradication; the world has been officially smallpox-free since 1980. Since then, however, vaccinations have ceased — meaning a mutated Mpox virus would encounter generations with no immunity. In that case, it might become necessary to rapidly develop drugs to combat the disease — especially since the mortality rate is relatively high among children and pregnant women.

“In the search for new therapeutics, our findings could be very helpful,” agree Utz Fischer and Clemens Grimm. The complex provides numerous docking sites for potential inhibitors and is well-suited for drug screening — the systematic search for new medicinal compounds.