Entomopathogenic fungi such as Metarhizium robertsii occupy a peculiar niche in the microbial world. Unlike bacteria that are ingested by their hosts, these fungi attack directly through the insect cuticle, germinating on the outer surface, penetrating the chitinous armor, and then proliferating in the open circulatory system of the insect. That journey demands a constant stream of secreted proteins, known as effectors, which are dispatched into the host to neutralize defenses and reconfigure host physiology in ways that favor fungal growth. Effectors that cross kingdom boundaries, moving from a fungal cell into an insect body, face an unusually harsh transit. They must fold correctly, avoid degradation, and remain soluble and secretion-competent in environments that differ radically from the fungal cytoplasm. How fungi safeguard their effectors through this transition has been a persistent gap in the molecular understanding of fungal pathogenesis, and it is precisely the gap that the new study set out to fill.

The team’s attention settled on MrNIS1, an effector previously associated with the virulence of Metarhizium, and on the question of what physical state the protein must occupy to survive its journey. The answer turned out to hinge on a post-translational modification: the attachment of an acetyl group to the lysine residue at position 24 of the protein. This acetylation, the researchers found, governs both the stability of MrNIS1 and its ability to be secreted. Crucially, the modification is not a passive chemical accident but the product of a regulated enzymatic cycle. MrNIS1 physically associates with MrKAT1, an acetyltransferase that installs the acetyl mark, and with MrSIR2, a deacetylase that can remove it. The presence of paired writing and erasing enzymes implies that the fungus actively manages the acetylation state of its effector, treating lysine 24 as a molecular switch whose position determines whether MrNIS1 remains a competent weapon or degrades into uselessness.

With the regulatory logic established, the researchers traced what acetylated MrNIS1 actually does once it reaches the insect interior, and here the protein revealed its dual nature. The first target is the insect immune system, specifically the cascade that culminates in prophenoloxidase activation. In insects, prophenoloxidase is the zymogen at the heart of melanization, the process by which invading pathogens are encased in dark, toxic melanin deposits and immobilized. Melanization is one of the most rapid and potent humoral defenses available to an insect lacking adaptive antibodies, and blocking it is a high-value objective for any pathogen. The study shows that MrNIS1 binds directly to the serine protease SPS, a component of the activation cascade, and by doing so prevents the chain of proteolytic events that would otherwise activate prophenoloxidase. The consequence is a suppressed melanization response, leaving the fungus freer to proliferate in the hemolymph without facing the melanin wall that healthy insects throw around invaders.

The second target lies far from immunity, in the endocrine machinery of development. MrNIS1 also interacts with ERP, an evolutionarily conserved cholesterol transporter. Cholesterol trafficking is a prerequisite for the biosynthesis of ecdysteroids, the molting hormones that orchestrate insect development, timing each larval stage and the transitions between them. By disrupting the function of ERP, MrNIS1 interferes with ecdysteroid production, and the downstream effects are visible in the host: larval growth is impaired and cuticle synthesis, which depends on properly timed hormonal cues, is compromised. In effect, a single fungal protein reaches into two disconnected physiological systems, one defensive and one developmental, and degrades both. That a secreted effector can achieve this dual targeting with a single polypeptide chain underscores how economical fungal virulence strategies can be, and it suggests that other cross-kingdom effectors may harbor similarly layered functions awaiting discovery.

Perhaps the most striking finding is how generalizable the second interaction is. The MrNIS1-ERP interaction proved to be functionally conserved across three of the largest insect orders: Lepidoptera, the moths and butterflies; Coleoptera, the beetles; and Diptera, the flies. Conservation across such distantly related insects indicates that the cholesterol transporter retains structural features that the fungal effector can recognize regardless of host lineage, which is exactly the property a broad-spectrum pathogen would need. For a fungus like Metarhizium robertsii, whose natural host range spans many insect species, an effector that works against a conserved developmental component multiplies its utility. From an applied perspective, this breadth is equally significant, because biocontrol agents are most valuable when they can act across the diverse pest species that attack crops, and an effector with cross-order activity is a ready-made template for such breadth.

The regulatory system itself also travels well between fungal species. When the researchers expressed MrNIS1 heterologously in Beauveria bassiana, another insect-pathogenic fungus widely used in commercial biocontrol, the KAT1/SIR2 acetylation-deacetylation system proved conserved and functional in its new host. MrNIS1 behaved there as what the authors describe as a portable virulence module, significantly enhancing the pathogenic efficacy of the recipient fungus. This experiment carries a double implication. Scientifically, it shows that the acetylation-dependent mechanism for preserving effector competence is not an idiosyncrasy of Metarhizium but a conserved strategy within insect-pathogenic fungi. Practically, it demonstrates that virulence factors can be transferred between fungal biocontrol agents along with their regulatory machinery, opening a rational engineering path: rather than selecting for virulence by trial and error, developers could install defined effector modules and their accompanying modification systems into established biocontrol strains.

The conceptual weight of the study lies in its reframing of lysine acetylation. Best known as a regulator of chromatin structure and enzyme activity within cells, acetylation here emerges as a guardian of protein competence outside the producing organism, a quality-control mark that preserves a secreted effector during the perilous transit between kingdoms. The interplay between MrKAT1 and MrSIR2 suggests a dynamic, tunable system rather than a one-time modification, raising questions the study leaves open: when during infection is the mark installed, whether the deacetylase acts to recycle or inactivate the effector, and how the fungus coordinates the acetylation state of its broader secretome. Answering these questions could reveal a general layer of regulation governing fungal effectors, with implications that extend beyond entomopathogenic fungi to plant pathogens and medically important species that likewise depend on secreted proteins to subvert their hosts.

For agriculture, the near-term significance is concrete. Metarhizium and Beauveria formulations are already deployed against pests as alternatives to chemical insecticides, but their adoption is limited by variable efficacy and slow kill relative to synthetic chemistry. Understanding that a single acetylated effector simultaneously suppresses immune melanization and disrupts ecdysteroid-driven development explains part of the mechanistic basis of fungal killing and identifies two host processes whose manipulation accelerates it. The conserved MrNIS1-ERP interaction offers a target for screening or engineering, while the demonstrated portability of the effector and its KAT1/SIR2 regulatory pair provides a direct method for building it into commercial strains. At the same time, the work is a reminder of the sophistication of cross-kingdom molecular conflict: a fungus has evolved, on one protein, a switch that keeps the weapon armed in transit, a grip on the host’s immune cascade, and a hand on the hormonal throttle of the host’s own growth. Decoding that switch, the authors argue, illuminates a conserved strategy of fungal pathogenesis and supplies tangible targets for the next generation of microbial biocontrol agents.

Subject of Research: Acetylation-dependent dual targeting of insect immunity and development by the fungal effector MrNIS1

Article Title: Acetylation enables a dual-targeting fungal effector to inhibit insect immunity and development

Article References: Wang, Y., Dong, Y., Zhang, Y., Wu, S., Yu, D., Yang, G., Gao, S., Yang, Y., Dong, S., Qin, L., & Huang, B. (2026). Acetylation enables a dual-targeting fungal effector to inhibit insect immunity and development. PLOS Pathogens, 22(10), e1014680. https://doi.org/10.1371/journal.ppat.1014680

Image Credits: AI Generated

DOI: 10.1371/journal.ppat.1014680

Keywords: Metarhizium robertsii, effector protein, lysine acetylation, insect immunity, melanization, prophenoloxidase, ecdysteroid biosynthesis, cholesterol transporter, biocontrol, Beauveria bassiana, fungal pathogenesis, cross-kingdom infection