Sticky Tags on a Viral Enzyme: SUMOylation Steers Herpesvirus Thymidine Kinase Into Neurons

Pseudorabies virus, an alphaherpesvirus that establishes lifelong latent infections in neurons, relies on a surprisingly delicate chemical modification of one of its core enzymes to switch between quiet persistence and destructive productive replication. A new study published in PLOS Pathogens reports that the viral thymidine kinase, a well-known virulence factor encoded by the UL23 gene, is decorated by small ubiquitin-like modifier proteins, or SUMO, and that this modification acts as a molecular address label directing the enzyme into the nucleus. The finding, from a research team led by Xuan Chen and Rui Zhang, reveals a post-translational regulatory layer that had previously gone unrecognized in alphaherpesvirus biology and points to a new vulnerability that could be exploited to blunt the pathogenicity of a virus with major consequences for the swine industry.

Thymidine kinase has long occupied a central place in herpesvirus research. The enzyme phosphorylates thymidine and related nucleosides, feeding nucleoside monophosphates into the viral DNA replication pathway. In non-dividing cells such as neurons, where the host cell’s own nucleotide pools and salvage pathways are limited, viral thymidine kinase becomes essential for efficient genome synthesis. It is also a key determinant of reactivation from latency, the process by which a dormant neuronal infection resumes productive replication and spreads to new hosts. Antiviral drugs such as acyclovir exploit the enzyme’s substrate specificity, because the kinase phosphorylates these drugs into toxic chain terminators preferentially in infected cells. Despite this importance, the mechanisms that govern where thymidine kinase accumulates inside an infected cell and how its activity is tuned during different phases of infection have remained poorly defined.

The new work began with the observation that thymidine kinase undergoes SUMOylation during authentic viral infection. SUMOylation is the covalent attachment of small ubiquitin-like modifier peptides to lysine residues of target proteins, a modification widespread in eukaryotic cells that typically alters protein interactions, stability, or localization without marking proteins for destruction the way ubiquitination often does. Viruses frequently co-opt or manipulate host SUMO machinery, and several viral proteins are themselves SUMOylated. The researchers showed that the pseudorabies virus thymidine kinase is conjugated not by a single SUMO paralog but by all three major forms found in mammalian cells, SUMO-1, SUMO-2, and SUMO-3, indicating that the modification is a genuine and broad feature of the protein’s life during infection rather than a rare or paralog-specific event.

Mapping the modification sites was the next step. Through mutational analysis, the team identified two lysine residues, K242 and K267, as the principal SUMO acceptor sites on the enzyme. These residues sit outside the catalytic core of the kinase, and when the investigators substituted them to prevent SUMO conjugation, they made a striking discovery: the mutant enzyme was neither destabilized nor catalytically impaired. Its stability inside infected cells was unchanged, and its kinase activity in vitro was preserved. What changed instead was where the protein went. Disruption of the SUMOylation sites markedly reduced the nuclear accumulation of thymidine kinase, shifting its distribution within the cell. In other words, SUMOylation functions not as an on-off switch for enzymatic activity but as a trafficking signal, ensuring that a fully competent enzyme reaches the compartment where it is needed.

The functional consequences of that mislocalization proved to be highly cell-type dependent. In dividing epithelial PK15 cells, a standard laboratory model for pseudorabies virus propagation, viruses carrying SUMOylation-deficient thymidine kinase replicated essentially normally. The modification was dispensable in this setting, presumably because epithelial cells provide abundant nucleotide pools and a permissive environment in which the cytoplasmic or diffuse distribution of the kinase suffices to support DNA synthesis. In neuronal N2a cells, however, the picture was entirely different. The SUMOylation-deficient virus replicated inefficiently in these non-dividing neuronal cells, demonstrating that nuclear delivery of thymidine kinase is a specific requirement for viral genome synthesis in the very cell type where the virus establishes its lifelong reservoir.

To connect this replication defect to the clinically and economically critical process of reactivation, the researchers employed an in vitro latency and reactivation model. They found that the SUMOylation-deficient virus was fully capable of establishing latent infection; the modification is not required for the virus to enter and persist in its dormant state. But when conditions were shifted to trigger reactivation, the mutant virus was markedly impaired, failing to resume productive replication efficiently. This dissociation between establishment and reactivation mirrors the known biology of thymidine kinase itself, which is dispensable for latency but essential for the resumption of lytic replication, and it places SUMOylation squarely within that reactivation pathway. The modification, in effect, arms the virus for exit from dormancy by ensuring the kinase is positioned correctly the moment replication restarts.

The in vivo significance of these findings was confirmed in a mouse infection model. Mice infected with the SUMOylation-deficient mutant virus experienced markedly attenuated disease compared with animals infected with wild-type pseudorabies virus. Viral loads in the brain were reduced, and histopathological lesions were milder, indicating that the loss of a single post-translational modification on one viral enzyme translates into a measurable loss of neuroinvasiveness and pathogenicity. Because pseudorabies virus is a close relative of the human alphaherpesviruses, including herpes simplex virus and varicella-zoster virus, the study raises the possibility that analogous SUMOylation events regulate thymidine kinase function in medically important human pathogens, although this remains to be tested directly.

Technically, the study illustrates the power of combining site-directed mutagenesis with cell-type-specific infection assays to dissect post-translational regulation. By separating effects on protein stability, catalytic activity, and subcellular localization, the authors were able to attribute the attenuation phenotype specifically to mislocalization rather than to a general loss of enzyme function. This distinction matters for antiviral strategy: a drug or therapeutic approach that blocks TK SUMOylation would not need to inhibit the kinase’s active site, which is shared in principle with cellular enzymes, but could instead prevent the enzyme from reaching its nuclear destination in neurons, selectively crippling the virus at the stage of neuronal replication and reactivation while sparing its replication in peripheral tissues.

The broader implication is that alphaherpesviruses encode their own localization control for a pivotal metabolic enzyme, and that this control is tuned to the biology of latency. A virus that must remain quiet for the lifetime of its host, yet be ready to reactivate at a moment’s notice, benefits from storing a fully active, correctly positioned replication factor in neurons. SUMOylation of thymidine kinase at K242 and K267 appears to provide exactly that readiness. As the authors conclude, the modification facilitates efficient neuronal replication, viral reactivation, and pathogenesis, and its disruption attenuates the virus without abolishing its ability to establish latency, a profile that could inform both vaccine design and the development of reactivation-blocking therapeutics. For a pathogen responsible for substantial losses in swine production worldwide, a single pair of lysine residues may prove to be an unexpectedly consequential target.

Subject of Research: SUMOylation-dependent nuclear localization of pseudorabies virus thymidine kinase and its role in neuronal replication and reactivation

Article Title: SUMOylation of pseudorabies virus thymidine kinase regulates its nuclear localization and facilitates neuronal replication and reactivation

Article References: Chen, X., Li, C., Xing, D., Song, J., Liu, Y., Liu, Y., Huang, Q., Kuang, Y., Li, X., Zhang, J., Zhang, Y., Zhang, N., Bai, R., Cheng, C., Yuan, J., Tang, J., & Zhang, R. (2026). SUMOylation of pseudorabies virus thymidine kinase regulates its nuclear localization and facilitates neuronal replication and reactivation. PLOS Pathogens, 22(10), e1014653. https://doi.org/10.1371/journal.ppat.1014653

Image Credits: AI Generated

DOI: 10.1371/journal.ppat.1014653

Keywords: pseudorabies virus, alphaherpesvirus, thymidine kinase, SUMOylation, post-translational modification, nuclear localization, neuronal replication, viral reactivation, latency, UL23, PLOS Pathogens, antiviral targets