Viral protein fragment offers compact scaffold for trimeric vaccines and antivirals

A short protein segment borrowed from a reovirus has emerged as a surprisingly capable tool for building trimeric vaccines and antiviral drugs. In a study published in Nature Communications, researchers at Xiamen University, working with Yi Guan of the University of Hong Kong, describe rFd1303, a compact trimerization motif derived from the σ1 attachment protein of reoviruses, members of the Reoviridae family. The team shows that this small domain can lock three identical protein chains together more reliably than the foldon domain from bacteriophage T4, the de facto standard in the field, and that it can be used to construct both vaccine immunogens and a broadly neutralizing antiviral against sarbecoviruses.

Trimerization motifs are short amino acid sequences that drive three separate polypeptide chains to associate into a single three-stranded assembly. They matter because many of the most important targets in vaccinology and virology are naturally trimeric. The spike proteins of coronaviruses and the hemagglutinin of influenza viruses both present their receptor-binding and fusion machinery as three copies arranged around a central axis. Presenting these antigens to the immune system in their correct oligomeric state preserves conformational epitopes that monomeric subunits lose, and it can dramatically improve the quality of the antibody response. The same logic applies to therapeutic proteins, where multivalency often increases binding avidity and functional potency.

The workhorse trimerization tag for the past two decades has been T4-Foldon, a 27-residue domain from the fibritin protein of bacteriophage T4. Foldon is effective but has limitations. It can leave residual populations of monomeric or misassembled product, it sometimes interferes with folding or purification of the fused cargo, and its yield benefits vary from one fusion partner to another. The Xiamen team searched the structural repertoire of the Reoviridae family for an alternative and identified a segment within the reovirus σ1 protein, an elongated fiber that naturally extends from the viral capsid and mediates attachment to host cells. The σ1 protein is itself an oligomeric fiber, and the researchers reasoned that a short stretch of it might carry the information needed to nucleate trimer assembly on its own.

That reasoning proved correct. The resulting motif, named rFd1303, is shorter than many conventional trimerization domains and, when genetically fused to the C-terminus of test proteins, drove efficient homo-trimerization in recombinant expression systems. Critically, the team observed that fusion constructs built on rFd1303 consistently gave higher recombinant protein yields than the equivalent T4-Foldon fusions. For vaccine manufacturing, where the cost of goods is often dominated by upstream expression and downstream purification, a tag that increases the amount of correctly assembled product per liter of culture is a meaningful practical advantage, not merely a structural curiosity.

To test whether the tag could support real immunogens, the researchers fused rFd1303 to the spike protein of SARS-CoV-2 and to influenza hemagglutinin, two of the most intensively engineered vaccine antigens of the past decade. When formulated and administered to mice, the rFd1303-fused immunogens elicited antibody responses comparable to those raised by the corresponding T4-Foldon-fused controls. This head-to-head comparison is important because it demonstrates that the new tag is not merely compatible with antigen display but functionally interchangeable with the established standard, while offering the yield benefit that motivated the search in the first place.

The therapeutic arm of the study pushed the platform further. The team used rFd1303 to engineer TriACE2-Ig, a trimeric fusion protein in which three copies of the human ACE2 receptor ectodomain are tethered to an immunoglobulin Fc region. The design exploits the fact that SARS-CoV-2 and its relatives enter cells by binding ACE2, so a soluble decoy receptor can sequester virus particles before they attach to cells. Trimerizing the decoy increases its apparent binding avidity for the viral spike, and the Fc region extends serum half-life and can engage immune effector functions. In stability testing, TriACE2-Ig remained intact after 30 days at room temperature, a property with obvious implications for distribution in settings where cold-chain infrastructure is limited.

Functionally, TriACE2-Ig neutralized a broad panel of ACE2-utilizing sarbecoviruses in vitro. The panel included 19 SARS-CoV-2 variants, the original SARS-CoV, and pangolin coronaviruses GD and GX, and the trimeric decoy showed higher neutralizing potency than a monomeric ACE2-Ig fusion. Breadth of this kind is valuable because it does not depend on the virus mutating away from a single antibody epitope; a decoy receptor mimics the conserved entry receptor itself, so escape would require the virus to abandon the very molecule it depends on for entry.

The team then moved into animal models. Syrian hamsters challenged with various SARS-CoV-2 variants received TriACE2-Ig intranasally, delivering the antiviral directly to the respiratory tract where the virus replicates. Treated animals showed markedly reduced viral loads, less virus-induced body-weight loss, and reduced lung pathology compared with controls. Notably, the treatment also decreased within-cage virus transmission, suggesting that lowering viral replication in index animals can interrupt onward spread in a household-like setting. If such effects translate to humans, an intranasal receptor decoy could serve both as a treatment for established infection and as a transmission-blocking adjunct during outbreaks.

The study positions rFd1303 as a versatile addition to the protein engineering toolbox rather than a single-purpose reagent. The same short tag that stabilizes vaccine antigens in their native trimeric state also assembles a multivalent therapeutic with real potency and unusual room-temperature stability. Several of the authors are coinventors on a Chinese patent application covering the domain and its applications, indicating that the team anticipates translational development. The work was supported by the National Natural Science Foundation of China and related funds, and the article is published open access with transparent peer review.

For the field, the message is that trimerization tags are not interchangeable commodities. The identity of the oligomerization domain influences expression yield, assembly fidelity, stability, and ultimately the manufacturability of both vaccines and biologics. A compact motif harvested from a relatively unassuming double-stranded RNA virus now offers an alternative to the foldon standard, and the TriACE2-Ig data suggest that the platform can carry a therapeutic candidate from design bench to animal efficacy in a single arc. As new sarbecoviruses continue to spill over from animal reservoirs, tools that enable rapid construction of both broadly protective antigens and broadly neutralizing decoys will remain in demand, and rFd1303 provides one such tool with its practical credentials already on display.

Subject of Research: Reoviridae-derived short trimerization domain for stabilizing homo-trimerization of vaccine immunogens and therapeutic proteins

Article Title: Reoviridae-derived short trimerization domain for stabilizing homo-trimerization of vaccine immunogens and therapeutic proteins

Article References: Wei, M., Bu, Q., Xiao, J., Yuan, L., Shi, J., Guo, H., Huang, Y., Sun, H., Wang, K., Li, Z., Lin, M., Lu, Z., Zheng, C., Chen, J., Gao, J., Wang, Z., Huang, Z., Bai, R., Liu, Y., … Yuan, Q. (2026). Reoviridae-derived short trimerization domain for stabilizing homo-trimerization of vaccine immunogens and therapeutic proteins. Nature Communications. https://doi.org/10.1038/s41467-026-77926-x

Image Credits: AI Generated

DOI: 10.1038/s41467-026-77926-x

Keywords: Reoviridae-derived, short, trimerization, domain, stabilizing, homo-trimerization, vaccine, immunogens, therapeutic, proteins, scientific research