In a significant breakthrough for virology and antiviral drug development, researchers have unveiled the high-resolution cryogenic electron microscopy (cryo-EM) structures of the RNA-dependent RNA polymerase (RdRp) of the Crimean–Congo haemorrhagic fever virus (CCHFV). This viral enzyme, also known as the L protein, is a critical component in the replication machinery of the virus, belonging to the Nairoviridae family, which is notorious for causing severe hemorrhagic fever with high fatality rates worldwide. The detailed molecular architecture provided by this study offers unprecedented insight into the enzymatic machinery that drives viral replication, opening new avenues for targeted therapeutic intervention against this deadly pathogen.
Crimean–Congo haemorrhagic fever virus presents a formidable public health challenge due to its capacity to induce severe disease characterized by fever, hemorrhagic manifestations, and multiorgan failure, often culminating in death. Despite its impact and the urgent need for effective treatments, no approved antiviral therapies currently exist to specifically inhibit CCHFV, leaving a critical gap in the global infectious disease armamentarium. The virus’s L protein, functioning as an RNA-dependent RNA polymerase, orchestrates the replication and transcription of the viral RNA genome, making it a prime target for antiviral drug development. However, the lack of detailed structural information had long hampered efforts to design molecules capable of disrupting its function.
Addressing this need, the new study presents cryo-EM structures of the CCHFV L protein in two functional states: an apo (unbound) conformation and an RNA-bound conformation, at resolutions of 2.62 Å and 2.53 Å, respectively. These near-atomic resolution models illuminate the individual domains of the polymerase, including distinct PA-, PB1-, and PB2-like regions—terms derived from their homology to the influenza virus polymerase subunits—within the polymerase core. This architectural framework elucidates how the enzyme’s domains are organized in three-dimensional space and how they cooperate dynamically during the viral RNA replication cycle.
The PA-like domain within the polymerase is implicated primarily in endonuclease activity required for cap-snatching, a mechanism by which the virus hijacks host mRNA caps to prime viral transcription. In contrast, the PB1-like domain harbors the canonical RNA polymerase catalytic motifs, facilitating the addition of nucleotides to the growing RNA strand. The PB2-like domain plays a multifaceted role related to binding and positioning of the viral RNA promoter, which is essential for initiating RNA synthesis. The new structures showcase the intricate spatial relationships of these domains and their concerted involvement in promoter recognition and polymerase function.
A particularly striking finding of the study is the detailed mechanism through which the polymerase recognizes viral RNA promoters. The resolved structures reveal how specific elements within the PB2-like domain engage the viral RNA promoter sequences, stabilizing the polymerase-RNA complex and orienting the RNA template for efficient replication. This precise promoter recognition mechanism is critical for ensuring the fidelity and efficiency of RNA synthesis, underscoring the specificity of viral RNA replication machinery as a potential pharmacological target.
Further insights emerged from comparing the apo state with the RNA-bound state of the polymerase, which revealed substantial conformational changes upon RNA engagement. These RNA-induced structural rearrangements stabilize key catalytic elements within the polymerase core, effectively “priming” the enzyme for RNA synthesis. This dynamic switching between inactive and active conformations highlights the complexity of the viral polymerase function and suggests strategies by which small molecules might lock the polymerase into nonfunctional states, thus abrogating viral replication.
The high resolution of the cryo-EM reconstructions allowed the researchers to model the active site with exceptional clarity, pinpointing residues involved in nucleotide binding and catalysis. Such detailed structural information is invaluable for rational drug design, enabling medicinal chemists to develop inhibitors that fit snugly into the polymerase’s active site or allosteric pockets, disrupting essential enzymatic activities. This is particularly compelling given the known challenges associated with inhibiting viral polymerases—enzymes that often share conserved motifs with cellular polymerases—highlighting the value of virus-specific structural insights.
This work represents the first atomic-level characterization of a nairovirus RNA polymerase, a landmark that bridges the gap between molecular virology and therapeutic innovation. It also lays a robust foundation for future studies aimed at screening or optimizing lead compounds that inhibit the CCHFV L protein based on structural data. The polymerase represents a bottleneck in the viral life cycle; therefore, inhibitors targeting this enzyme have high potential to effectively suppress viral replication and ameliorate disease outcomes.
Beyond immediate therapeutic implications, the structural information offers a rich resource for understanding evolutionary relationships within the order Bunyavirales, to which Nairoviridae belong. By comparing the CCHFV polymerase with those of other RNA viruses, researchers can trace conserved and unique features that underlie the functional diversity of viral RdRps, ultimately refining our understanding of viral replication mechanisms across diverse families.
Despite the novelty of these findings, further research is warranted to elucidate how polymerase inhibitors could function within the complex intracellular environment of infected cells. Structural and functional assays, combined with medicinal chemistry efforts, will be necessary to translate these atomic insights into clinically effective antiviral agents. Moreover, the role of additional viral and host cofactors in polymerase function and regulation remains an open field, promising to deepen comprehension of viral replication networks.
The new L protein structures also invite exploration of the interplay between CCHFV polymerase and host immune responses. Insights into polymerase structure-function relationships could inform strategies aiming not only to block replication but also to modulate innate immune recognition, potentially enhancing antiviral immunity or mitigating immunopathology associated with severe disease.
In summary, the elucidation of the Crimean–Congo haemorrhagic fever virus RNA polymerase architecture represents a transformative advance in the molecular virology of Nairoviridae. By providing a detailed molecular blueprint of the polymerase’s domains, promoter recognition, and RNA-induced activation, this study delivers crucial insights into the fundamental mechanisms driving viral replication. These findings fuel optimism for structure-guided development of polymerase inhibitors that may become the first targeted antivirals against this lethal hemorrhagic fever virus. As the global scientific community grapples with emerging infectious diseases, such advances underscore the indispensability of structural biology in combating viral threats.
Looking ahead, the enabling potential of cryo-electron microscopy is clearly demonstrated by this study, emphasizing the technique’s role in resolving complex viral enzymes at near-atomic resolution. This accomplishment showcases the importance of interdisciplinary collaboration among virologists, structural biologists, and drug developers to translate atomic insights into tangible medical advances. The hope is that these contributions will accelerate the development pipeline, ultimately yielding lifesaving therapies against Crimean–Congo haemorrhagic fever and related viral hemorrhagic fevers, diseases that continue to loom as global health threats.
The study reported here marks a milestone in efforts to outpace the molecular machinations of CCHFV by illuminating its replication apparatus in exquisite detail—a critical foundation for next-generation antiviral discovery. With expanding knowledge of viral polymerase structures and functions, the prospect of effective CCHFV therapeutics, once elusive, moves closer to reality, heralding a promising new chapter in the fight against this devastating pathogen.
Subject of Research:
Structure and function of Crimean–Congo haemorrhagic fever virus RNA-dependent RNA polymerase (L protein)
Article Title:
Structure of Crimean–Congo haemorrhagic fever virus RNA polymerase
Article References:
Wang, D., Yang, G. & Liu, B. Structure of Crimean–Congo haemorrhagic fever virus RNA polymerase. Nat Microbiol (2026). https://doi.org/10.1038/s41564-026-02319-1
Image Credits:
AI Generated
