Herpesvirus protein mimics host enzyme to balance infection and latency

Viruses are microscopic pathogens composed of genetic material housed within a protective protein shell. While they carry the blueprint required for producing additional viral proteins, they lack the equipment needed for the same. For this, they hijack host cells and rely on their machinery for multiplication.

Viruses have evolved multiple strategies that help them gain control of the host’s cellular processes and make the environment conducive to their survival and replication. One key strategy is mimicking the host’s regulatory factors that govern important cellular pathways. However, viral infection dramatically transforms the host’s cellular environment. Therefore, the regulatory mechanisms of viral factors are likely distinct from those of host factors.

Phosphorylation—the reversible addition of phosphate groups carried out by proteins known as kinases—acts as a switch that turns key cell cycle-related proteins on and off. Interestingly, some viral protein kinases in herpesviruses are structurally conserved and mimic human cyclin-dependent kinases (CDKs).

Herpes simplex virus 2 (HSV-2) causes skin and genital infections, meningitis, and neonatal diseases and often establishes a lifelong infection characterized by periods of latency and recurrent outbreaks. Understanding the mechanisms that regulate CDK mimicry by HSV-2 can aid the development of novel antiviral therapies and preventive vaccines.

To this end, Professor Yasushi Kawaguchi and Assistant Professor Naoto Koyanagi from the Division of Molecular Virology, Department of Microbiology and Immunology, Institute of Medical Science, The University of Tokyo, Japan, investigated the mechanisms underlying phosphorylation-mediated CDK mimicry by conserved herpesvirus protein kinases (CHPKs).

Providing further insight into their work, Prof. Kawaguchi explains, “We demonstrated that UL13, which is the CHPK of HSV-2, mimics the functional and regulatory mechanisms of CDKs. This mimicry by UL13 contributes not only to the suppression of lethal viral infection in the brain but also to the efficient recurrence of the virus. Our findings unveil part of the sophisticated strategy employed by the virus to ensure its persistence within the host.”

Their findings were published in the Proceedings of the National Academy of Sciences on April 16, 2025.

CDKs contain two structurally and functionally distinct lobes: the N- and C-lobes. The N-lobe contains a highly conserved motif across CDKs and CHPKs, in which phosphorylation of specific serine/threonine and tyrosine residues inhibits its catalytic activity.

The researchers examined whether UL13 was phosphorylated at the corresponding tyrosine residue in the conserved motif in cells infected with either wild-type HSV-2, UL13-deficient HSV-2, or HSV-2 UL13-Y162F, in which UL13 Tyr-162 was replaced with phenylalanine.

An antibody targeting tyrosine-phosphorylated UL13 reacted strongly with lysates obtained from wild-type-infected cells. Conversely, there was no reaction in lysates from mock-infected control cells, cells infected with UL13-deficient HSV-2 or HSV-2 UL13-Y162F. Notably, phosphorylation was more pronounced at 24 hours post-infection than 12 hours, suggesting that tyrosine phosphorylation by UL13 occurred later during HSV-2 infection.

Further, mutational analyses revealed that phosphomimetic mutation of tyrosine in UL13 reduced the phosphorylation of all UL13 substrates (proteins phosphorylated by UL13), suggesting that phosphorylation downregulates its catalytic activity. Similar effects were noted in other subfamilies of herpesviruses as well.

Additionally, phosphomimetic mutation at tyrosine in HSV-2 UL13 downregulated viral replication and pathogenicity in the brain of mice injected with HSV-2, during the active lytic phase (when new viral particles are released). Conversely, in guinea pigs infected with HSV-2, phosphorylation of UL13 tyrosine residue was required to establish recurrent infection following the latent phase. These findings suggest that CDK mimicry by UL13 may regulate both acute lytic and latent HSV-2 infections in vivo.

Overall, the study highlights a novel regulatory mechanism of CDK mimicry that may help herpesviruses coexist and expand within host cells while balancing host survival with viral persistence throughout the host’s lifespan.

Prof. Kawaguchi concludes, “Insights into the regulatory mechanisms of CHPK kinase activity may lead to a better understanding of how pathogenicity is controlled across different herpesviruses. Furthermore, the corresponding tyrosine residue in the conserved motif of CDKs and CHPKs is also conserved in viral kinases encoded by poxviruses, suggesting poxviruses might have also evolved CDK regulatory mimicry.”