S-palmitoylation, also called S-acylation, is the covalent attachment of a saturated 16-carbon palmitate to a cysteine residue through a thioester bond. Unlike other lipid modifications, the thioester linkage is intrinsically unstable, which makes the modification reversible and dynamically regulated by two antagonistic enzyme families: palmitoyl acyltransferases (PATs), which carry a conserved DHHC catalytic motif and add palmitoyl groups, and depalmitoylases, which hydrolyze the thioester bond and strip acyl groups away. In humans, 23 PATs have been identified and many are implicated in disease, but the enzymology of S-acylation in plants remains comparatively underexplored. Arabidopsis encodes 24 PATs with distinct subcellular localizations, and S-palmitoylated proteins have been catalogued in rice, maize and soybean, yet validated enzyme-substrate pairs in plant-virus systems have been scarce, limiting mechanistic insight into how this modification shapes infection.

The research team, led by Yan Xie and Xueping Zhou of Zhejiang University together with colleagues, began by establishing where TYLCCxV C4 resides inside plant cells. When C4 fused to green fluorescent protein was expressed in N. benthamiana leaf cells, fluorescence accumulated predominantly at the cytoplasm and plasma membrane, with weak nuclear signal. Subcellular fractionation experiments reinforced the picture: C4 co-sedimented with a plasma membrane marker in the membrane-enriched pellet fraction, while free GFP stayed in the soluble supernatant. Bioinformatic prediction tools suggested that C4 lacks a transmembrane domain but carries two candidate lipid modifications, N-myristoylation at glycine 2 and S-acylation at cysteine 4, hinting that a fatty anchor rather than a hydrophobic helix might explain the membrane association.

To test that hypothesis directly, the researchers deployed the biotin-switch assay, a technique in which hydroxylamine selectively cleaves thioester bonds and the newly freed cysteines are tagged with biotin for detection. A strong S-acylation signal appeared in hydroxylamine-treated samples of C4 but not in untreated controls. Quantitative mass spectrometry using Q-Exactive liquid chromatography-tandem mass spectrometry then pinpointed the modification to cysteine 4. When the team substituted that cysteine with serine, generating a mutant called C4(C4S), the S-acylation signal vanished entirely. The consequences were striking: the mutant redistributed to the cytoplasm and nucleus, and its protein accumulation collapsed to roughly a tenth of wild-type levels, even though semi-quantitative RT-PCR showed that transcript abundance was unchanged. Treating plants with 2-bromopalmitate, a chemical inhibitor of palmitoylation, produced the same dual effect, driving C4 out of the membrane and reducing its steady-state abundance without touching its mRNA.

The functional stakes became clear in infection experiments. When C4 or the palmitoylation-deficient C4(C4S) was expressed from a potato virus X vector, plants receiving wild-type C4 developed systemic upward leaf curling and stem elongation by 14 days post-inoculation, while plants receiving the mutant showed only mild chlorotic symptoms indistinguishable from the empty-vector control. Western blots confirmed that C4 protein levels and PVX coat protein accumulation were comparable across treatments, ruling out differences in expression or vector fitness. More importantly, the team engineered an infectious TYLCCxV clone carrying the same C4S mutation. Plants inoculated with the mutant virus developed dramatically milder symptoms than those infected with wild-type TYLCCxV, and Southern blot analysis revealed weak viral DNA signals at 15 days that diminished further by 30 days. S-acylation of a single cysteine, in other words, is a major determinant of geminiviral pathogenicity and accumulation.

Having established that the modification matters, the researchers went hunting for the enzymes that control it. Screening four candidate palmitoyl acyltransferases from N. benthamiana with a yeast split-ubiquitin assay, they found that NbPAT4 and NbPAT6 interacted with C4. They focused on NbPAT4 and confirmed the interaction in plant cells using bimolecular fluorescence complementation, which reconstituted yellow fluorescent protein signal at the cytomembrane, and co-immunoprecipitation, which pulled down NbPAT4 specifically with C4 but not with free GFP. Genetic evidence followed: in CRISPR-generated NbPAT4 knockout plants, TYLCCxV induced only mild leaf curling and accumulated less viral DNA, whereas plants overexpressing NbPAT4 developed more severe symptoms, including vein swelling and leaf clustering, and carried higher viral loads. Biotin-switch measurements placed the enzyme at the chemistry itself: C4 S-acylation levels fell to about half of wild type in knockout plants and rose to nearly double in overexpression lines.

Palmitoylation is a two-way street, so the team also screened 19 candidate depalmitoylases, including four acyl-protein thioesterases and fifteen alpha/beta hydrolase domain-containing proteins, for interactors of C4. Only one emerged: NbABHD6, a 274-amino-acid enzyme containing a conserved alpha/beta hydrolase domain with a catalytic triad of serine 147, aspartate 212 and histidine 241, mirroring the architecture of the mammalian ABHD17 family. Yeast split-ubiquitin, bimolecular fluorescence complementation and co-immunoprecipitation all confirmed a specific interaction with C4. Functionally, co-expression of NbABHD6 reduced C4 S-acylation levels to roughly 0.6 of control values and cut C4 protein accumulation by half, again without altering transcript levels. When the three catalytic residues were simultaneously mutated to alanine, the resulting NbABHD6(mSDH) mutant still bound C4 but lost all ability to reduce its S-acylation or abundance, demonstrating that the depalmitoylase activity, not mere physical association, drives the effect.

The fate of depalmitoylated C4 proved to be destruction by the ubiquitin-proteasome system. Treatment with the proteasome inhibitor MG132 increased C4 accumulation, while inhibitors of autophagy, E64d and 3-methyladenine, had no effect, indicating that C4 is normally turned over through the 26S proteasome. When NbABHD6 was co-expressed, C4 levels dropped sharply in vehicle-treated leaves, but MG132 partially rescued the protein, and the catalytically dead mutant had no such effect. The authors note that even with the proteasome blocked, NbABHD6 still reduced C4 below control levels, hinting at additional regulatory mechanisms consistent with C4’s multifunctional nature. They also observed that NbABHD6 did not markedly change C4’s membrane localization, likely because the myristoylation site at glycine 2 provides a second, independent lipid anchor, a reminder that subcellular localization alone cannot reliably report S-acylation status.

The findings sit within a growing appreciation that S-acylation is a battleground in plant-microbe encounters. Rice stripe virus disrupts the S-palmitoylation of host Remorin 1 and shunts it into autophagy; potato mop-top virus hijacks the acylated stress sensor HIPP26 for long-distance movement; and the C4 protein of beet severe curly top virus uses its own S-acylation to engage the receptor kinase CLAVATA 1. In mammals, an elegant palmitoylation cycle on STAT3, catalyzed by DHHC7 and reversed by APT2, governs inflammatory signaling, illustrating how dynamic acylation can act as a molecular switch. The new study extends this logic to plant antiviral immunity and, notably, identifies only the second defined plant depalmitoylase-viral substrate pair, after the recent report that Arabidopsis ABAPT3 targets the C4 protein of beet severe curly top virus.

The authors frame their results as a host-pathogen enzymatic tug-of-war aligned with the recently proposed concept of apoplastic interactive balance. Early in infection, TYLCCxV exploits the host’s own S-acylation machinery: NbPAT4 palmitoylates C4, anchoring it at the plasma membrane, stabilizing the protein and amplifying viral accumulation. The plant counters by deploying NbABHD6, whose depalmitoylase activity destabilizes C4 and channels it into proteasomal degradation, suppressing infection. The equilibrium between these opposing enzymes sets the stoichiometry of C4 acylation and, with it, the trajectory of disease. Open questions remain, including whether C4 recruits specific E3 ubiquitin ligases and how S-acylation crosstalks with myristoylation, phosphorylation and ubiquitination. But the identification of NbPAT4 and NbABHD6 as antagonistic regulators offers a concrete enzymatic axis, and potentially a target for engineering crop resistance to one of agriculture’s most damaging virus families.

Subject of Research: Reversible S-palmitoylation of the geminiviral C4 protein and its regulation by host palmitoylation enzymes during plant-virus interaction

Article Title: Reversible S-palmitoylation of C4 protein encoded by TYLCCxV orchestrates geminiviral pathogenesis

Article References: Xie, Y., Zhao, M., Liu, X., Yan, J., Yang, W., Chen, Y., Yang, M., Wang, X., Fu, S., & Zhou, X. (2026). Reversible S-palmitoylation of C4 protein encoded by TYLCCxV orchestrates geminiviral pathogenesis. Stress Biology, 6(1), Article 35. https://doi.org/10.1007/s44154-026-00308-2

Image Credits: AI Generated

DOI: 10.1007/s44154-026-00308-2

Keywords: S-palmitoylation, geminivirus, C4 protein, TYLCCxV, palmitoyl acyltransferase, depalmitoylase, NbPAT4, NbABHD6, Nicotiana benthamiana, plant-virus interaction, 26S proteasome, post-translational modification

The protein at the center of the story is WD40-REPEAT 5a, or WDR5a, a structural backbone component of the Arabidopsis COMPASS-like complex. This complex is the plant equivalent of a histone methyltransferase assembly first characterized in yeast and animals, and its job is to deposit trimethylation marks on lysine 4 of histone H3, a modification universally associated with active gene transcription. When WDR5a is present and functional, the COMPASS-like complex maintains proper H3K4me3 levels across the genome, keeping thousands of genes in a transcriptionally permissive state. When WDR5a is lost, those marks erode, and genes that depend on them fall silent. Previous work had shown that WDR5a is constantly turned over by the ubiquitin-proteasome system, but the identity of the proteasome subunit responsible for its degradation, and the developmental consequences of that degradation, remained unknown.

To find the missing link, the researchers turned to a classic tool of molecular biology: the yeast two-hybrid screen. By baiting the screen with WDR5a, they pulled out an unexpected partner, REGULATORY PARTICLE AAA-ATPASE 2a, or RPT2a. RPT2a is no ordinary protein; it is one of six AAA-ATPases that form a ring at the entrance of the 26S proteasome’s regulatory particle, unfolding ubiquitinated substrates and threading them into the proteolytic core for destruction. Earlier studies had established that RPT2a is essential for meristem maintenance in Arabidopsis, and that mutations in the gene cause pleiotropic developmental defects, including a characteristic halted-root phenotype. The new finding suggested that at least part of RPT2a’s developmental influence might flow through a specific substrate: WDR5a itself.

The interaction was not a fleeting artifact of the screen. The team confirmed the physical association between RPT2a and WDR5a in plant cells, and then asked what the interaction means functionally. Their experiments showed that RPT2a promotes the degradation of WDR5a through the 26S proteasome pathway. When proteasomal activity is compromised, or when RPT2a function is reduced, WDR5a accumulates. Conversely, the degradation of WDR5a depends on its prior tagging with ubiquitin, the small protein flag that directs substrates to the proteasome. The researchers traced that tagging to a Cullin4-based ubiquitin ligase complex. CUL4, a scaffold protein, partners with the adaptor DNA Damage Binding protein 1A, or DDB1A, to recognize WDR5a and recruit the enzymatic machinery that attaches ubiquitin chains. This CUL4-DDB1A complex, they found, is what targets WDR5a for ubiquitination in vivo.

Perhaps the most technically satisfying part of the study is the mapping of the ubiquitination sites themselves. By analyzing which lysine residues on WDR5a receive ubiquitin marks, the team identified lysine 31 and lysine 211 as the principal targets. These residues sit on exposed surfaces of the WD40 beta-propeller structure, consistent with their accessibility to the CUL4-DDB1A ligase. Mutating these lysines stabilizes the protein, providing direct genetic evidence that they are the functional degradation signals. The finding echoes a striking parallel from human biology: the X-linked mental retardation gene CUL4B was previously shown to target the mammalian WDR5 ortholog for ubiquitylation, regulating neuronal gene expression. The Arabidopsis work suggests that the strategy of controlling H3K4 methylation by destroying a COMPASS scaffold protein is an evolutionarily conserved theme, deployed independently in plants and animals.

With the degradation pathway established, the researchers connected it to the phenotype that matters most to a plant: when it flowers. WDR5a’s influence on H3K4me3 extends to FLOWERING LOCUS C, or FLC, the master floral repressor of Arabidopsis. FLC encodes a transcription factor that binds directly to the chromatin of FLOWERING LOCUS T (FT) and SUPPRESSOR OF OVEREXPRESSION OF CO 1 (SOC1), the two genes whose activation is required for the floral transition. As long as FLC is expressed, FT and SOC1 stay off and the plant remains vegetative. The new study shows that RPT2a-mediated degradation of WDR5a modulates H3K4me3 at FLC and thereby its expression level, establishing an inverse relationship: when WDR5a is degraded and FLC transcription falls, FT and SOC1 rise, and flowering is promoted. When WDR5a is stabilized, FLC stays high, FT and SOC1 stay low, and flowering is delayed.

The genetic evidence supports this model at every node. Plants with reduced RPT2a function accumulate WDR5a, show altered H3K4me3 patterns, misexpress FLC, and display measurable shifts in flowering time under both long-day and short-day conditions. Manipulating WDR5a levels phenocopies the effects, and the expression changes in FT and SOC1 track faithfully with the FLC changes, exactly as expected from the well-characterized FLC-FT/SOC1 regulatory module. In other words, a proteasome subunit once viewed simply as a generic component of the cell’s waste-disposal system turns out to act as a specific, substrate-selective regulator of a developmental switch, funneling protein-turnover information into the epigenetic control of a single decisive gene.

The broader significance of the work lies in what it reveals about how plants integrate proteostasis with chromatin regulation. The 26S proteasome has long been known to shape plant development, with subunit mutants showing defects in meristem maintenance, hormone signaling, and stress responses. But most of those effects were attributed to the wholesale turnover of signaling proteins such as transcription factors and repressors. The new study adds a subtler layer: by degrading a chromatin scaffold, the proteasome can reconfigure the histone modification landscape itself, changing not just the abundance of individual regulators but the accessibility of entire genomic regions. A recent companion study from another group showed that the RPT2a-MET1 axis controls TERMINAL FLOWER1 and inflorescence meristem determinacy, suggesting that RPT2a may act as a hub connecting protein degradation to multiple chromatin systems, from DNA methylation to histone methylation, across diverse developmental contexts.

There are also practical implications. Flowering time is a major determinant of yield in crops, and the COMPASS-like complex has already been implicated in panicle branching and flowering in rice. If the RPT2a-WDR5a-CUL4/DDB1A pathway identified in Arabidopsis is conserved in crop species, it could offer breeders a new set of targets for tuning flowering schedules, whether to escape seasonal stresses, synchronize flowering for hybrid seed production, or adapt varieties to shifting climates. The identification of specific ubiquitination sites on WDR5a is particularly appealing from this perspective, since those residues define a molecular interface that could in principle be modified to alter protein stability without abolishing function.

Many questions remain. What signals trigger the CUL4-DDB1A ligase to attack WDR5a at a given moment? Is the degradation pathway responsive to environmental cues such as photoperiod or temperature, which are the dominant natural inputs to flowering time? And does RPT2a recognize WDR5a directly, or does it simply provide the proteasomal gate through which the ubiquitinated protein passes? Answering these questions will require connecting the biochemical pathway to the circadian and vernalization circuits that plants use to sense the seasons. What is already clear, however, is that the decision to flower is not made solely at the level of gene transcription. It is also made, moment by moment, at the mouth of the proteasome, where a molecular gatekeeper decides how long a chromatin scaffold survives, and with it, how much longer a plant will wait to bloom.

Subject of Research: Proteasome-mediated degradation of the histone H3K4 methyltransferase component WDR5a and its role in regulating flowering time in Arabidopsis

Article Title: RPT2a-mediated degradation of WDR5a regulates flowering time in Arabidopsis

Article References: Liu, J., Liu, Y.-Y., Wu, J., Zhang, Y.-T., Yang, F., Du, Q., & Chen, L.-Y. (2026). RPT2a-mediated degradation of WDR5a regulates flowering time in Arabidopsis. Plant Cell Reports, 45(10), Article 296. https://doi.org/10.1007/s00299-026-03966-8

Image Credits: AI Generated

DOI: 10.1007/s00299-026-03966-8

Keywords: RPT2a, WDR5a, 26S proteasome, COMPASS-like complex, H3K4me3, flowering time, Arabidopsis, FLC, CUL4-DDB1A, ubiquitination, epigenetics, plant development