Breakthrough Cryo-EM Study Maps Herpes Replication Protein in New Detail

Researchers have resolved the first high-resolution structures of the herpes simplex virus origin-binding protein (OBP), revealing unexpected mechanisms of viral DNA replication and uncovering multiple new targets for antiviral drug development. 

In a major advance for antiviral research, an international team has determined the first high-resolution structures of the herpes simplex virus OBP, a critical player in viral DNA replication that has resisted structural analysis for over 40 years.

Published in Nucleic Acids Research, the study reveals unexpected mechanisms behind HSV-1 replication and highlights several promising targets for the development of next-generation antiviral therapies.

Using cryo-electron microscopy (cryo-EM), scientists from Karolinska Institutet, the University of Gothenburg, and the Center for Structural Systems Biology in Hamburg captured OBP in action at resolutions as fine as 2.8 Å. These detailed snapshots show the protein in multiple functional states - bound to viral DNA origin sequences and interacting with an ATP analog - offering new insight into the earliest steps of herpesvirus replication.

This is particularly important because current HSV-1 treatments almost exclusively target the viral DNA polymerase, and we're seeing increasing resistance to these drugs, especially in immunocompromised patients. OBP represents an entirely new target that acts even earlier in the viral lifecycle, before the polymerase is recruited.

Martin Hällberg, Study Corresponding Author, The Department of Cell and Molecular Biology, Karolinska Institutet

Unexpected Architecture Reveals Regulatory Mechanisms

The study uncovered several structural surprises.

Rather than forming the expected configuration, OBP assembles as a head-to-tail dimer with a novel regulatory twist: the extreme C-terminus of each protein molecule threads through its partner, positioning itself near the ATP-binding site. This architecture explains a long-standing puzzle as to why deleting this region boosts helicase activity but impairs overall replication.

The C-terminus appears to act as an intrinsic brake on helicase activity. When the viral single-stranded DNA-binding protein ICP8 binds to OBP, it likely releases this brake, coordinating the transition from origin recognition to active DNA unwinding.

Emil Gustavsson, Study First Author, Centre for Structural Systems Biology

Multiple Drug Targets Identified

The structural data revealed several potential drug-binding sites. One of the most promising is the OBP’s unique DNA-binding motif, which the virus uses to identify replication starting points. Disrupting this interaction could stop replication before it begins. The dimer interface, essential for OBP’s stability and function, is another viable target.

Researchers also highlighted the ICP8-binding region, which plays a regulatory role in helicase activity, as a potential site for therapeutic intervention. Perhaps most compelling is the unusual configuration of the ATP-binding pocket (distinct from other known helicases), which could enable the design of highly specific inhibitors.

We've essentially provided a molecular blueprint for drug design. These diverse targeting options could help overcome resistance mechanisms and potentially even prevent viral reactivation from latency, and, perhaps as important, we will now be able to precisely address regulatory features of viral DNA replication during latency and lytic replication.

Per Elias, Study Co-Author, University of Gothenburg

Implications Beyond Current Therapies and Implications for Cancer Patients

With HSV-1 infecting roughly 70 % of the global population, the need for new treatments is urgent.

Resistance to existing therapies is especially problematic in immunocompromised patients, such as those undergoing chemotherapy or bone marrow transplants, where herpesvirus reactivation is common and dangerous. In such cases, doctors often must turn to less effective or more toxic options like foscarnet.

Beyond its role in acute infection, HSV-1 and other neurotropic herpesviruses like varicella-zoster are also being studied for their possible links to neurodegenerative diseases. Drugs that prevent reactivation from latency could eventually serve as preventative treatments for these conditions.