An international research team has achieved a significant breakthrough by determining the first high-resolution structures of the herpes simplex virus origin-binding protein (OBP). This protein is essential for the replication of the virus and has evaded structural analysis for over four decades. The findings, published in the journal Nucleic Acids Research, uncover novel mechanisms through which the virus initiates DNA replication and present multiple potential targets for the development of new antiviral drugs.
Structural Insights from Cryo-EM Technology
Utilizing advanced cryo-electron microscopy (cryo-EM), researchers from Karolinska Institutet, the University of Gothenburg, and the Centre for Structural Systems Biology in Hamburg captured detailed images of OBP at resolutions reaching up to 2.8 Å. The high-resolution structures reveal the protein in various functional states, bound to viral DNA origin sequences and complexed with an ATP analogue. These insights provide an unprecedented view of the early stages of herpes virus replication.
According to corresponding author Martin Hällberg from the Department of Cell and Molecular Biology at Karolinska Institutet, this research is critical. “Current HSV-1 treatments primarily target the viral DNA polymerase, and we are observing increasing resistance to these drugs, especially among immunocompromised patients,” Hällberg stated. “OBP represents an entirely new target that acts even earlier in the viral lifecycle, before the polymerase is recruited.”
Unveiling New Mechanisms and Drug Targets
The research unveiled several unexpected findings regarding OBP. Contrary to previous assumptions, the protein forms a head-to-tail dimer with a unique regulatory mechanism. The extreme C-terminus of each OBP molecule interlocks with its partner, positioning itself near the ATP-binding pocket. This configuration resolves a long-standing question about why deleting this region enhances helicase activity but diminishes overall replication efficiency.
Emil Gustavsson, the first author from the Centre for Structural Systems Biology, explained, “The C-terminus appears to function as an intrinsic brake on helicase activity. When the viral single-stranded DNA-binding protein ICP8 binds to OBP, it likely releases this brake, facilitating the transition from origin recognition to active DNA unwinding.”
The high-resolution structures provide several promising sites for antiviral drug development. The unique DNA-binding motif that identifies viral origins is essential for replication, making it a potential target. Additionally, the dimer interface, critical for protein stability and function, offers another avenue for intervention. The ICP8-binding region, which regulates helicase activity, could be targeted to disrupt the coordination between viral proteins.
Perhaps most intriguingly, the structures highlight an unusual configuration of the ATP-binding pocket that differs from other helicases. This distinction may lead to the development of highly specific inhibitors. “We’ve essentially provided a molecular blueprint for drug design,” stated Per Elias from the University of Gothenburg. “These diverse targeting options could help overcome resistance mechanisms and potentially even prevent viral reactivation from latency.”
Approximately 70% of the global population carries HSV-1, which can cause symptoms ranging from cold sores to severe complications such as encephalitis. The urgency for alternative treatments is underscored by the emergence of drug-resistant strains, particularly in immunocompromised individuals. Patients undergoing chemotherapy or bone marrow transplantation face increased challenges, as treatment-induced immune suppression often triggers the reactivation of latent herpesviruses.
When drug resistance occurs among these patients, clinicians may have to resort to less effective or more toxic alternatives, such as foscarnet, highlighting the critical need for new therapeutic options. Additionally, there is growing interest in the potential role of neurotropic herpesviruses, such as herpes simplex and varicella zoster virus, in contributing to neurodegenerative diseases. Novel drugs aimed at preventing the reactivation of herpesviruses from latency may provide new strategies for preventing and treating these debilitating conditions.
This research was funded by the Swedish Research Council, the Swedish Cancer Foundation, and the Helmholtz Association. Cryo-EM data were collected at the 3D-EM facility at Karolinska Institutet and the Centre for Structural Systems Biology in Hamburg. The detailed structural coordinates, cryo-EM maps, and raw data have been made publicly available in the Protein Data Bank, Electron Microscopy Data Bank, and the Electron Microscopy Public Image Archive for the benefit of the wider scientific community.
