The parasite faces relentless genotoxic pressure during its life inside human red blood cells. As it digests host hemoglobin, free heme is released and oxidized from the ferrous to the ferric state, generating reactive oxygen species that inflict a range of DNA lesions, including double-strand breaks. Compounding this vulnerability, Plasmodium species apparently lack the classical nonhomologous end joining pathway, leaving homologous recombination as the primary, and likely exclusive, route for repairing broken chromosomes. That singular reliance makes the recombination machinery an attractive target for antimalarial drugs, because the parasite cannot fall back on an alternative repair system.

PfBlm sits at the heart of this machinery. Previous work has shown that the helicase is upregulated in response to DNA damage, interacts directly with key recombination proteins such as PfRad51 and PfMre11, and that overexpression of the wild-type protein enhances parasite survival under genotoxic stress while a helicase-dead mutant fails to rescue repair defects. Parasites lacking Pfblm show telomere lengthening, relaxed subtelomeric chromatin, and upregulation of subtelomeric var genes, pointing to additional roles in telomere maintenance and in suppressing uncontrolled diversification of the virulence genes that help the parasite evade the immune system. Earlier studies also demonstrated that the small-molecule inhibitor ML216, which blocks PfBlm’s helicase activity, sensitizes parasites to DNA-damaging agents and synergizes with frontline antimalarials.

The Hyderabad team, led by Mrinal Kanti Bhattacharyya, asked whether the molecular chaperone PfHsp90 governs PfBlm’s stability and function. Hsp90 is an ATP-dependent chaperone whose client proteins include many stress-response and genome-maintenance factors. In P. falciparum, Hsp90 is essential for parasite growth, and the geldanamycin analog 17-AAG has been validated as a specific inhibitor of PfHsp90 activity in vitro. The group had previously established that PfRad51 and the sirtuin histone deacetylase PfSir2A are bona fide PfHsp90 clients, and that combining a PfHsp90 inhibitor with a PfRad51 inhibitor produces strong synergistic antimalarial effects in both drug-sensitive 3D7 and multidrug-resistant Dd2 parasite strains.

To test whether PfBlm also depends on PfHsp90, the researchers first probed for a physical interaction. A yeast two-hybrid assay, in which PfHsp90 was fused to a GAL4 DNA-binding domain and PfBlm to a GAL4 activation domain, showed reporter gene expression consistent with an interaction, although the lack of growth on more stringent adenine-selection plates indicated the association is relatively weak. Critically, co-immunoprecipitation from P. falciparum 3D7 parasite lysates using an anti-PfHsp90 antibody pulled down PfBlm, while preimmune IgG did not, confirming that the two proteins associate within the parasite itself. Notably, PfBlm was also detected in the unbound supernatant fraction, suggesting that only a portion of the cellular PfBlm pool is engaged with PfHsp90 at any given moment.

The team then examined what happens to PfBlm when chaperone function is lost. They expressed PfBlm in a temperature-sensitive yeast strain in which both endogenous HSP90 genes are deleted and viability is maintained by a temperature-sensitive hsp82 allele. At the permissive temperature of 25 degrees Celsius, PfBlm remained stable, but after four hours at the restrictive temperature of 37 degrees Celsius, PfBlm levels dropped by roughly 75 percent, as quantified from western blots of three independent experiments. No such temperature-dependent decline occurred in a wild-type yeast strain, confirming that the depletion resulted specifically from the loss of Hsp90 chaperone activity rather than heat stress itself.

Parallel experiments in the parasite told a consistent story. Treating synchronized ring-stage cultures with 1.7 micromolar 17-AAG for twelve hours significantly reduced PfBlm abundance at the late trophozoite stage. When the proteasome inhibitor MG132 was added alongside 17-AAG, PfBlm levels were restored, and MG132 alone actually raised steady-state PfBlm, indicating that the helicase normally turns over through the proteasome and that PfHsp90 is required to protect it from proteasomal degradation. Semi-quantitative RT-PCR showed that PfBLM transcript levels were not downregulated by 17-AAG; in fact they increased, implying transcriptional upregulation in response to chaperone inhibition. The protein loss, therefore, is a post-transcriptional effect driven by degradation, not reduced gene expression.

The decisive question was whether this chaperone-client relationship matters for DNA repair. Because PfHsp90 inhibition also depletes PfRad51, blocking the chaperone alone cannot selectively reveal PfBlm-dependent repair defects. The researchers therefore designed a drug-combination experiment using a sensitive PCR-based repair assay. Synchronized parasites were exposed to ultraviolet irradiation at 100 joules per square meter to induce genome-wide double-strand breaks, and repair kinetics were tracked by amplifying a long 7,200-base-pair genomic fragment, normalized against a short 269-base-pair fragment, at time points before damage, immediately after, and at 12 and 24 hours of recovery. The team first identified doses of each drug that were individually harmless to repair: 510 nanomolar 17-AAG left repair fully intact, 250 nanomolar ML216 caused no inhibition, and 500 nanomolar ML216 inhibited repair by only about 16 percent.

The combination, however, was dramatically more damaging than either drug alone. Co-treatment with 510 nanomolar 17-AAG and 500 nanomolar ML216 produced roughly 60 percent inhibition of double-strand break repair, far exceeding the approximately 16 percent expected from a purely additive effect. This statistically significant excess indicates a synergistic interaction, consistent with PfHsp90 activity contributing to PfBlm abundance and function. The growth-inhibition assays reinforced the point: in the presence of the IC50 concentration of 17-AAG, the IC50 of ML216 fell from 1.579 micromolar to 0.041 micromolar, a more than 38-fold potentiation of the helicase inhibitor’s activity. The interpretation is that 17-AAG shrinks the pool of functional PfBlm by destabilizing it, so a much lower dose of ML216 is sufficient to disable the residual active protein, pushing cellular PfBlm activity below the threshold needed for repair.

The findings extend the known clientele of PfHsp90 among recombination factors and suggest a broader regulatory role for the chaperone in parasite genome biology. PfHsp90 stabilizes the pro-recombinogenic PfRad51 while also supporting the anti-recombinogenic PfBlm, which restrains var gene diversification and preserves subtelomeric chromatin, and it additionally modulates var silencing through PfSir2A. This balancing act allows the parasite to maintain core genome integrity while permitting the controlled recombination that underlies antigenic variation, a key virulence strategy. Intriguingly, a similar physical association between human Hsp90 and the Bloom helicase has been reported in immortalized human cells, where the chaperone modulates Blm’s helicase activity on telomeric DNA substrates in a substrate-dependent manner, hinting at an evolutionarily conserved chaperone-helicase partnership. Whether such context-dependent regulation operates in the malaria parasite remains to be determined. The authors propose that the PfHsp90-PfBlm axis could be exploited to develop new chemotherapeutic strategies against malaria, with simultaneous targeting of chaperone and helicase offering a way to block DNA repair and limit parasite survival, particularly valuable at a time when resistance to existing antimalarials continues to spread.

Subject of Research: Chaperone-dependent stabilization of the PfBlm DNA repair helicase and dual-inhibitor antimalarial strategy in Plasmodium falciparum

Article Title: Inhibition of Plasmodium DNA double strand repair by the combined action of 17-AAG and ML216

Article References: Inhibition of Plasmodium DNA double strand repair by the combined action of 17-AAG and ML216. (n.d.). https://doi.org/10.3389/fpara.2026.1896831

Image Credits: AI Generated

DOI: 10.3389/fpara.2026.1896831

Keywords: Plasmodium falciparum, malaria, PfBlm helicase, PfHsp90, DNA double-strand break repair, homologous recombination, 17-AAG, ML216, molecular chaperone, RecQ helicase, antimalarial drug targets, proteasomal degradation

The cellular defense in question centers on the structural maintenance of chromosomes complex 5/6, a multi-subunit assembly better known for its roles in chromosome biology, including DNA repair and the organization of chromosomal architecture. In recent years, several studies have established that SMC5/6 is not merely a housekeeping complex but also a potent antiviral restriction factor. When episomal DNA viruses, whose genomes persist as circular DNA molecules separate from host chromosomes, attempt to establish infection, SMC5/6 can be recruited to these foreign genomes and impose a repressive chromatin state on them, effectively locking the viral DNA into a transcriptionally silent configuration. Hepatitis B virus, papillomaviruses, and other DNA viruses have all been shown to contend with this restriction, and each has evolved countermeasures, underscoring how fundamental this defense is to the outcome of infection.

A critical insight from prior work is that SMC5/6-mediated silencing of extrachromosomal DNA depends on the physical relocation of the complex to promyelocytic leukemia nuclear bodies, membrane-less nuclear structures long associated with antiviral activity. This recruitment is mediated by SLF2, a protein that acts as a dedicated localization factor, tethering SMC5/6 to these nuclear compartments. Because promyelocytic leukemia nuclear bodies are widely regarded as sites of intrinsic antiviral defense, their integrity is a recurring target of viral attack. Many viruses, including human cytomegalovirus, actively disrupt these structures during infection, dispersing their constituent proteins throughout the nucleoplasm. Given this established interplay, the authors of the new study asked a direct question: what happens to the SMC5/6 complex itself when human cytomegalovirus enters a cell and begins dismantling the very compartments where the complex exerts its antiviral effect?

The answer, arrived at through careful time-course experiments at the onset of infection, was unexpected in one respect and revealing in another. The researchers found that the core components of the SMC5/6 complex were not downregulated by the virus; their abundance remained essentially unchanged as infection proceeded. Instead, the team observed a distinct and reproducible decrease in the protein levels of SLF2, and this loss correlated with the complete dispersal of the SMC5/6 complex away from promyelocytic leukemia nuclear bodies. In other words, the virus did not need to eliminate the restriction complex itself; it needed only to remove the factor responsible for delivering that complex to the compartments where silencing takes place. The strategic logic is elegant: by cutting the tether rather than the payload, the virus prevents the antiviral machinery from ever reaching the incoming viral genomes.

Determining how SLF2 was being eliminated required the researchers to dissect the mechanism of its depletion. Two experimental observations provided important clues. First, the degradation of SLF2 was also observed after infection with ultraviolet-inactivated virus, particles that had been rendered incapable of replicating their genomes or expressing new viral genes. Second, the loss of SLF2 could not be blocked by cycloheximide, a drug that halts cellular protein synthesis and thereby prevents the production of any newly made viral proteins. Together, these findings pointed to a striking conclusion: the factor responsible for SLF2 degradation must be a structural component that is physically packaged into the virus particle and delivered into the cell at the moment of entry, rather than a protein synthesized later in the replicative cycle. Such virion-delivered effectors are a hallmark of herpesvirus biology, allowing these viruses to act against host defenses before their own genes have even begun to be expressed.

To pinpoint the culprit among the virion’s structural proteins, the researchers conducted a targeted screen of candidate tegument proteins, the layer of viral proteins that sits between the viral envelope and the capsid and is released into the cell upon entry. After excluding a requirement for the well-characterized tegument protein pp71, encoded by the UL82 gene, the screen identified UL35 as the protein that is both required and sufficient for the proteasomal degradation of SLF2. The proteasome is the cell’s principal machinery for destroying damaged or unwanted proteins, and viruses routinely hijack it to eliminate host factors that stand in their way. The demonstration that UL35 co-opts this pathway to remove SLF2 places the protein in a growing family of viral effectors that weaponize cellular protein quality control against innate and intrinsic immunity.

Genetic confirmation of UL35’s role came from experiments with a recombinant human cytomegalovirus carrying a stop codon within the UL35 gene region, rendering the protein nonfunctional. In cells infected with this mutant virus, SLF2 protein levels remained stable, and the consequences for the virus were substantial: the mutant displayed a clear defect in the onset of immediate early gene expression, the very first wave of viral transcription that must occur for infection to proceed. This finding connects the molecular mechanism directly to viral fitness, showing that the removal of SLF2 is not a side effect of infection but a functional prerequisite for productive replication. Without UL35, the virus struggles to switch on its genes, precisely the outcome one would expect if the SMC5/6 silencing system were left intact and operational.

Microscopic and biochemical analyses of the mutant infection provided a vivid picture of what happens when the viral antagonist is absent. In cells infected with the UL35-deficient virus, the researchers observed enhanced recruitment of the SMC5/6 complex to promyelocytic leukemia nuclear bodies, and, critically, an overall increase in the association of parental viral genomes, the incoming DNA molecules that have not yet replicated, with these SMC5/6-containing nuclear compartments. The incoming viral genomes, in effect, were being corralled into the very structures where episomal DNA is silenced. This observation provides direct evidence that the SLF2-dependent localization pathway acts on cytomegalovirus genomes during the earliest phase of infection and that UL35 exists, at least in part, to prevent this sequestration from happening.

The final piece of mechanistic confirmation came from a complementary loss-of-function approach. When the researchers depleted SLF2 using small interfering RNAs in cells infected with the UL35-deficient virus, the phenotype was reversed: the accumulation of SMC5/6 at nuclear bodies and the association of viral genomes with those compartments were abolished. This epistasis experiment demonstrates that SLF2 is the relevant host target through which UL35 exerts its effect, and it firmly establishes UL35 as a novel viral antagonist of the SLF2-dependent arm of the SMC5/6 restriction system. The logic of the pathway is now complete: SLF2 delivers SMC5/6 to promyelocytic leukemia nuclear bodies, where the complex can silence incoming episomal viral genomes; UL35, delivered as a structural component of the infecting virion, triggers proteasomal degradation of SLF2 before this localization can occur, thereby sparing the viral genomes from silencing and enabling immediate early gene expression to begin on schedule.

Beyond its immediate mechanistic contribution, the study carries broader implications for virology and antiviral research. It adds human cytomegalovirus to the list of DNA viruses known to counter the SMC5/6 pathway, reinforcing the idea that this complex represents a widely deployed, evolutionarily conserved barrier against episomal DNA. It also highlights the strategic importance of virion-delivered tegument proteins, which allow herpesviruses to neutralize intrinsic immunity within hours of entry, before any adaptive or even transcriptional responses can mount. From a translational perspective, the SLF2-SMC5/6 axis and its viral antagonists offer potential points of intervention: compounds that stabilize SLF2, block UL35 function, or enhance the recruitment of SMC5/6 to viral genomes might tip the balance toward silencing of incoming cytomegalovirus genomes, particularly relevant for immunocompromised patients in whom reactivation and de novo infection remain serious clinical problems. As with other recently discovered antiviral restriction systems, the discovery that a major herpesvirus dedicates a structural protein to dismantling this pathway is itself testimony to how consequential the defense is, and it suggests that further exploration of SMC5/6 biology may yield fresh strategies against a virus that, despite decades of research, still lacks a vaccine and remains a leading cause of congenital disease.

Subject of Research: How human cytomegalovirus protein UL35 antagonizes SMC5/6-mediated silencing of viral genomes by degrading SLF2

Article Title: Human cytomegalovirus antagonizes SMC5/6 driven genome silencing via UL35 instituted proteasomal degradation of SLF2

Article References: Montagner, F., Scherer, M., Stegmann, C., Hahn, F., Ploil, C., Einsiedler, C. S., Brosch, E. K., Maeki, T., Ramani, R., Andrée-Busch, N., Brinkmann, M. M., & Stamminger, T. (2026). Human cytomegalovirus antagonizes SMC5/6 driven genome silencing via UL35 instituted proteasomal degradation of SLF2. PLOS Pathogens, 22(10), e1014709. https://doi.org/10.1371/journal.ppat.1014709

Image Credits: AI Generated

DOI: 10.1371/journal.ppat.1014709

Keywords: human cytomegalovirus, SMC5/6, SLF2, UL35, PML nuclear bodies, proteasomal degradation, viral tegument, intrinsic immunity, episomal DNA silencing, immediate early gene expression, herpesvirus, antiviral restriction factors

To understand why this finding matters, it helps to begin with the biology of protein turnover. Inside every cell, proteins that are no longer needed, damaged, or dangerous are tagged with small marker proteins called ubiquitins. Enzymes assemble these tags into chains, and the way the chains are linked determines the tagged protein’s fate. K48-linked polyubiquitin chains, in which ubiquitin molecules are connected through a specific lysine residue, act as a molecular kiss of death: they flag the protein for delivery to the proteasome, the cell’s garbage disposal, where it is chopped into fragments. Deubiquitinating enzymes, or DUBs, do the opposite. They snip ubiquitin tags off proteins, rescuing them from degradation. Among the largest DUB family is the ubiquitin-specific proteases, or USPs, and when one of these enzymes removes death marks from a protein that promotes cancer, the effect can be dramatic: the oncoprotein accumulates, and the cell drifts further toward uncontrolled growth.

CIAP2, also known as baculoviral IAP repeat containing protein 2 and a member of the inhibitor of apoptosis protein family, was already known to be overexpressed in hepatocellular carcinoma and to function there as an oncogene. What remained incompletely understood was how tumor cells manage to keep CIAP2 levels so high. The research team approached this question systematically. Rather than guessing which of the many ubiquitin-specific proteases might be responsible, they screened USPs that could plausibly regulate CIAP2 expression and identified USP45 as a key regulator that enhances CIAP2 levels in hepatocellular carcinoma cells. That screening result set the stage for a deeper interrogation of what USP45 is doing in these tumors and whether its activity has real consequences for cancer behavior.

Bioinformatic analysis of clinical data provided the first hint that USP45 is more than a laboratory curiosity. The researchers found that USP45 is overexpressed in hepatocellular carcinoma and, critically, that elevated USP45 is associated with worse patient survival. In other words, the enzyme is not merely present in these tumors; its abundance tracks with the outcome that matters most to patients. This kind of correlation does not by itself prove causation, but it supplied the rationale for the functional experiments that followed. If USP45 truly helps drive the disease, then removing it from cancer cells should blunt their malignant behavior in measurable ways.

That is precisely what the team observed. When the researchers knocked down USP45 in two hepatocellular carcinoma cell lines, SNU449 and SK-Hep-1, the consequences were broad and consistent. The cells lost viability, and their ability to form colonies in culture was impaired. The cell cycle was disrupted, with cells accumulating in an arrested state rather than progressing smoothly through division. Apoptosis, the programmed self-destruction that cancer cells typically resist, increased. Beyond growth and survival, the cells’ metastatic potential also weakened: migration and invasion, the two behaviors that allow a tumor to spread beyond its original site, were attenuated. Finally, sphere formation, an in vitro assay that reflects the self-renewal capacity associated with cancer stem cells, was compromised, suggesting that USP45 supports not just bulk tumor growth but also the stem-like subpopulation that often fuels relapse and treatment resistance.

The downstream signaling changes reinforced this picture. USP45 deficiency suppressed the NF-κB signaling pathway, a major pro-survival and pro-inflammatory cascade that many cancers exploit to keep growing under hostile conditions. At the same time, loss of USP45 reduced the expression of key transcription factors involved in cancer cell stemness, the molecular program that endows a subset of tumor cells with the self-renewal and differentiation plasticity of stem cells. Taken together, these results indicate that USP45 sits upstream of at least two malignant programs in hepatocellular carcinoma cells: the inflammatory survival signaling that keeps cells alive and the stemness program that maintains a reservoir of aggressive, therapy-resistant cells.

The mechanistic core of the study lies in the physical and functional relationship between USP45 and CIAP2. Using co-immunoprecipitation, the researchers showed that USP45 interacts with CIAP2 in hepatocellular carcinoma cells, and endogenous co-immunoprecipitation experiments, which detect interactions between the proteins as they naturally occur in cells rather than in artificial overexpression systems, confirmed that this partnership is physiological. Functionally, USP45 removes ubiquitination from CIAP2 and enhances its stability, meaning the enzyme directly extends the lifespan of the oncoprotein inside the cell. The team then narrowed the mechanism further with ubiquitin mutant assays. By testing cells co-transfected with Flag-tagged CIAP2, HA-tagged USP45, and either wild-type ubiquitin or mutant ubiquitin lacking the K48 or K63 linkage sites, they demonstrated that USP45 specifically targets K48-linked polyubiquitin chains on CIAP2. In plain terms, USP45 strips off precisely the ubiquitin marks that would otherwise condemn CIAP2 to the proteasome, sparing the K63-linked chains that serve other signaling roles.

A crucial rescue experiment established that CIAP2 is not merely one target among many but the functionally relevant mediator of USP45’s oncogenic activity. When the researchers overexpressed CIAP2 in USP45-deficient hepatocellular carcinoma cells, the malignant traits that had been lost came back: the cells regained the aggressive behaviors that USP45 depletion had taken away. This epistasis-style logic, in which restoring a downstream factor rescues a phenotype caused by losing an upstream factor, is a strong indication that the USP45–CIAP2 axis is the operative pathway rather than a side effect of some other change. It also suggests that the amount of CIAP2 available to the tumor cell, which USP45 controls by editing its ubiquitin tags, is a limiting factor for the malignant state.

The final piece of evidence came from an animal model. In a hepatocellular carcinoma xenograft mouse model, in which human cancer cells are grown as tumors in immunodeficient mice, depletion of USP45 inhibited tumor growth and disrupted various malignant characteristics of the tumors. This in vivo result matters because cell culture experiments, however thorough, cannot fully capture the complexity of a growing tumor with its nutrient gradients, stromal interactions and selective pressures. The xenograft data indicate that the USP45–CIAP2 mechanism is not an artifact of the culture dish but operates in the context of an actual expanding tumor, strengthening the case that the pathway is relevant to real disease progression.

Collectively, the findings position USP45 as an oncogene in hepatocellular carcinoma and define its mechanism of action with unusual precision: USP45 binds CIAP2, removes K48-linked polyubiquitin chains from it, stabilizes the protein, and thereby sustains the survival, proliferation, motility and stem-like behavior of liver cancer cells, with the axis confirmed both in cultured cells and in xenograft tumors. The study was approved by the Ethics Committee of The Affiliated Hospital of Putian University and was supported by the Fujian Provincial Natural Science Foundation and the Joint Funds for the Innovation of Science and Technology of Fujian Province. For the field, the work adds USP45 to the growing list of deubiquitinating enzymes implicated in cancer and nominates the USP45–CIAP2 pair as a potential therapeutic axis. Because the enzyme–substrate relationship is defined at the level of a specific ubiquitin linkage, it offers a concrete biochemical handle for future drug discovery efforts aimed at destabilizing CIAP2 in liver tumors, although translating such mechanistic insight into clinical interventions will require the usual long road of validation and development.

Subject of Research: The role of the deubiquitinating enzyme USP45 in stabilizing CIAP2 and promoting hepatocellular carcinoma progression

Article Title: USP45 Promotes hepatocellular carcinoma progression by regulating the ubiquitination and stability of CIAP2

Article References: Jiang, X., Chen, J., Lin, W., Huang, K., Wang, L., & Lin, J. (2026). USP45 Promotes hepatocellular carcinoma progression by regulating the ubiquitination and stability of CIAP2. Cellular and Molecular Life Sciences. https://doi.org/10.1007/s00018-026-06409-5

Image Credits: AI Generated

DOI: 10.1007/s00018-026-06409-5

Keywords: hepatocellular carcinoma, USP45, CIAP2, deubiquitinating enzymes, ubiquitination, K48-linked polyubiquitin, proteasomal degradation, NF-kB signaling, cancer stem cells, xenograft mouse model, liver cancer, oncoprotein stability