The study, led by Prasanth Viswanathan and colleagues, focused on a protein called myeloid cell leukemia-1, or MCL1. MCL1 belongs to a family of proteins that act as molecular bodyguards, blocking the self-destruct program known as apoptosis. It is an oncogene, meaning that when it is amplified or overactive it drives cancer, and tumors with high MCL1 levels are notoriously resistant to chemotherapy. Because of its clinical importance, MCL1 inhibitors are already being developed and tested in trials, making it an attractive target for drug designers. Previous work had shown that PEL cell lines, the laboratory models of this lymphoma, cannot survive without MCL1, but the details of how MCL1 protects these virus-transformed cells remained murky.

To dig deeper, the team turned to a systems biology approach, mapping the web of genetic interactions surrounding MCL1 in PEL cells. This unbiased strategy surfaced a surprise: MCL1 appeared to function in the same pathway as a protein called MARCHF5, a membrane-anchored E3 ubiquitin ligase residing in the mitochondria. MARCHF5 was not an obvious suspect. It is best known for maintaining mitochondrial homeostasis and for tuning antiviral signaling pathways, not for regulating apoptosis. The genetic interaction hinted, however, that the two proteins cooperate in keeping PEL cells alive, and the researchers set out to test that idea directly.

The first experiment was decisive. Using CRISPR gene editing, the team knocked out the MARCHF5 gene in PEL cell lines. Even though MCL1 was still present and functional, the loss of MARCHF5 triggered a significant increase in apoptosis. In other words, MCL1 alone was not enough to keep the cells alive; they also needed MARCHF5. This was a striking result because it established that a mitochondrial ubiquitin ligase, a protein class usually discussed in the context of mitochondrial quality control, plays an essential anti-apoptotic role in these virus-transformed tumor cells.

MARCHF5 is an enzyme, and enzymes can often be disabled by breaking their catalytic machinery. The researchers probed which activities of MARCHF5 were required for its survival function. Two features proved essential: its E3 ligase activity, the enzymatic ability to attach ubiquitin tags to target proteins, and its capacity to dimerize, or pair up with another MARCHF5 molecule. When either of these activities was disrupted, the protein could no longer protect PEL cells from apoptosis. This mechanistic detail matters because it suggests the survival function of MARCHF5 is not a passive structural role but an active enzymatic process, one that could in principle be targeted with drugs.

What was MARCHF5 actually doing? The clue came from an unexpected direction. When the researchers removed MARCHF5, or when they treated the cells with inhibitors of the 26S proteasome, the cellular machine that destroys ubiquitin-tagged proteins, one particular protein accumulated: NOXA. NOXA is a pro-apoptotic member of the same protein family as MCL1, and its job is essentially to antagonize MCL1 and push the cell toward self-destruction. Intriguingly, the levels of MCL1 itself did not change under these conditions. Instead, MARCHF5 appeared to be keeping NOXA in check, and without MARCHF5 the lethal signal built up unchecked.

The team then asked whether NOXA was truly the pro-apoptotic signal that MCL1 and MARCHF5 exist to neutralize. Several lines of evidence converged on that conclusion. Endogenous reciprocal co-immunoprecipitation experiments, which pull protein complexes out of cells without artificial tagging, showed that MARCHF5 and NOXA are found together in the same protein complex, consistent with a direct physical relationship. Even more telling, when the researchers knocked out the NOXA gene on its own, the PEL cells gained a fitness advantage, suggesting that NOXA exerts a constant, damaging pressure on these tumor cells that must be continuously suppressed for survival.

The most convincing test came from a genetic rescue experiment. If NOXA accumulation is the reason that MARCHF5-deficient cells die, then deleting NOXA should save them. That is exactly what the researchers observed: knocking out NOXA restored cell viability in PEL lines that had lost MARCHF5. Together, these experiments provide the mechanistic link that underlies the original genetic interaction between MCL1 and MARCHF5. The model that emerges is elegant: MARCHF5, using its ubiquitin ligase activity, induces the degradation of NOXA, the natural enemy of MCL1, thereby reinforcing MCL1’s pro-survival function and allowing the KSHV-transformed cells to evade apoptosis.

This finding adds a new layer to the biology of KSHV-associated malignancies. Primary effusion lymphoma arises when the virus reprograms B cells into rapidly proliferating, apoptosis-resistant tumor cells that typically grow as lymphomas in body cavities. The virus achieves this in part by manipulating the host cell’s apoptotic machinery, and the MCL1-MARCHF5 axis now appears to be a central node in that manipulation. The fact that both proteins are required, and that they act through a defined substrate, NOXA, gives researchers a clearer picture of the survival circuitry that these tumors depend on. It also raises the possibility that similar cooperation between MCL1 and mitochondrial ubiquitin ligases operates in other cancers where MCL1 is amplified.

From a therapeutic standpoint, the implications are considerable. MCL1 inhibitors are an active area of drug development precisely because MCL1 amplification drives chemoresistance in many cancers. The new work suggests that combination therapies targeting both MCL1 and MARCHF5, or approaches that prevent MARCHF5 from degrading NOXA, could be more effective than hitting either protein alone, since the two oncogenes reinforce each other’s function. Disabling MARCHF5’s ligase activity would strip away one of the defenses that keeps NOXA suppressed, potentially sensitizing tumor cells to MCL1 inhibition or to existing chemotherapy regimens. As the authors note, this newly appreciated interaction between the MCL1 and MARCHF5 oncogenes may prove useful in improving the design of combination therapies for KSHV malignancies, offering a rational strategy against a cancer that has so far proven difficult to treat.

Subject of Research: How the mitochondrial ubiquitin ligase MARCHF5 cooperates with MCL1 to inhibit apoptosis in KSHV-transformed primary effusion lymphoma cells

Article Title: The Mitochondrial ubiquitin ligase MARCHF5 cooperates with MCL1 to inhibit Apoptosis in KSHV-transformed primary effusion lymphoma cell lines

Article References: Viswanathan, P., Bersonda, J. R., Gill, J., Navarro, A., Farrar, A. C., Caldarera, T., Murphy, A., Dunham, D., Boehme, K. W., & Manzano, M. (2026). The mitochondrial ubiquitin ligase MARCHF5 cooperates with MCL1 to inhibit apoptosis in KSHV-transformed primary effusion lymphoma cell lines. PLOS Pathogens, 22(10), e1014665. https://doi.org/10.1371/journal.ppat.1014665

Image Credits: AI Generated

DOI: 10.1371/journal.ppat.1014665

Keywords: KSHV, primary effusion lymphoma, MCL1, MARCHF5, NOXA, apoptosis, ubiquitin ligase, mitochondria, CRISPR, oncogene, viral oncology, combination therapy