Biology subjects

Bisio, H.

Publications and source records attributed to Bisio, H..

7 recordsLinked to original sources

bioRxiv2025

Elucidating the interaction network of one of the largest icosahedral capsids in the virosphere

Giant viruses challenge traditional boundaries of virology with their large particle sizes, complex genomes, and unique replication strategies. Yet, despite its 750 nm diameter and incorporation of dozens of proteins, mimivirus virion retains an icosahedral symmetry, a trait often associated with smaller viruses. The functional roles and interactions of most proteins composing such complex icosahedral particles remain elusive. Here, we dissect the spatial and functional organization of mimivirus morphogenesis by integrating bioinformatics, genetics and interactomics. We performed protein clustering using a structure-informed approach integrating AlphaFold models with sequence information to classify and functionally annotate the ORFan-rich mimivirus proteome. To map the protein-protein interaction network during morphogenesis, we employed endogenous tagging and co-immunoprecipitation coupled to mass spectrometry. This strategy revealed distinct interaction modules associated with the virion membrane, nucleoid and viral factory compartments. Comparative analyses with other icosahedral and non-icosahedral giant viruses uncovered conserved assembly nodes and virion shape-specific adaptations. Our findings shed light on the global organization of mimivirus virion biogenesis and highlight the evolutionary plasticity of viral morphogenetic networks within the Nucleocytoviricota.

microbiology↗

bioRxiv2025

Giant DNA viruses encode a hallmark translation initiation complex of eukaryotic life

In contrast to living organisms, viruses were long thought to lack protein synthesis machinery and instead depend on host factors to translate viral transcripts. Here, we discover that giant DNA viruses encode a distinct and functional IF4F translation initiation complex to drive protein synthesis, thereby blurring the line between cellular and acellular biology. During infection, eukaryotic IF4F on host ribosomes is replaced by an essential viral IF4F that regulates viral translation, virion formation, and replication plasticity during altered host states. Structural dissection of viral IF4F reveals that the mRNA cap-binding subunit mediates exclusive interactions with viral mRNAs, constituting a molecular switch from translating host to viral proteins. Thus, our study establishes that viruses express a eukaryotic translation initiation complex for protein synthesis, illuminating a series of evolutionary innovations to a core process of life.

microbiology↗

bioRxiv2025

Melbournevirus encodes a shorter H2B-H2A doublet histone variant that forms structurally distinct nucleosome structures.

Unique among viruses, some giant viruses utilize histones to organize their genomes into nucleosomes. Melbournevirus encodes a distinct H2B-H2A histone doublet variant in addition to the canonical H4-H3 and H2B-H2A doublets. This viral histone variant has a truncated H2B portion and its amino acid sequence deviates from that of the main viral H2B-H2A throughout the entire coding region. It is less abundant than the main H2B-H2A doublet, is essential for melbournevirus fitness, and is conserved in all Marseilleviridae. The cryo-EM structure of a nucleosome-like particle reconstituted with this H2B-H2A variant and viral H4-H3 reveals that only 90 base pairs of DNA are stably bound, significantly less than in eukaryotic nucleosomes and viral nucleosomes made with the main fused viral histone doublets. The reduced ability to bind DNA can be attributed to structural differences between variant and main H2B-H2A. Variant melbournevirus nucleosomes are less stable, possibly aiding rapid genome unpacking to initiate gene expression. Our results highlight the remarkable propensity of giant viruses to appropriate the utility of histones for their specialized purposes.

molecular biology↗

bioRxiv2024

Nucleocytoviricota viral factories are transient organelles made by phase separation

Phase separation is a widespread mechanism in viral processes, mediating replication, host manipulation and virion morphogenesis. The phylum Nucleocytoviricota encompasses diverse and ubiquitous viruses, including Poxviridae, the climate-modulating Emiliania huxleyi virus and other so-called Giant Viruses. Cytoplasmic members of this phylum form viral factories but their nature has remained unresolved. Here, we demonstrate that these viral factories are formed by liquid-liquid phase separation. We prove that mimivirus viral factories are formed by multilayered phase separation, orchestrated by at least two scaffold proteins. To extend these findings across the phylum Nucleocytoviricota, we developed a bioinformatic pipeline to predict scaffold proteins based on a conserved molecular grammar, despite major primary sequence variability. Scaffold candidates were validated in Marseilleviridae and Poxviridae, highlighting a role of H5 as a scaffold protein in the vaccinia virus. Finally, we provide a repertoire of client proteins of the nucleus-like viral factory of mimivirus and demonstrate important sub-compartmentalization of functions, including those related to the central dogma. Overall, we reveal a new mechanism for an organelle to deploy nuclear-like functions entirely based on phase separation and re-classified phylum Nucleocytoviricota viral factories as biomolecular condensates.

microbiology↗

bioRxiv2023

Knockout of GMC-oxidoreductase genes reveals functional redundancy in mimivirus

The mimivirus 1.2Mb genome was shown to be organized into a nucleocapsid-like genomic fiber encased in the nucleoid compartment inside the icosahedral capsid (1). The genomic fiber protein shell is composed of a mixture of two GMC-oxidoreductase paralogs, one of them being the main component of the glycosylated layer of fibrils at the surface of the virion (2). In this study, we determined the effect of the deletion of each of the corresponding genes on the genomic fiber and the layer of surface fibrils. First, we deleted the GMC-oxidoreductase the most abundant in the genomic fiber, and determined its structure and composition in the mutant. As expected, it was composed of the second GMC-oxidoreductase and contained 5- and 6-start helices similar to the wild-type fiber. This result led us to propose a model explaining their coexistence. Then, we deleted the GMC-oxidoreductase the most abundant in the layer of fibrils to analyze its protein composition in the mutant. Second, we showed that the fitness of single mutants and the double mutant were not decreased compared to the wild-type viruses in laboratory conditions. Third, we determined that deleting the GMC-oxidoreductase genes did not impact the glycosylation or the glycan composition of the layer of surface fibrils, despite modifying their protein composition. Since the glycosylation machinery and glycan composition of members of different clades are different (3, 4), we expanded the analysis of the protein composition of the layer of fibrils to members of the B and C clades and showed that it was different among the three clades and even among isolates within the same clade. Taken together, the results obtained on two distinct central processes (genome packaging and virion coating) illustrate an unexpected functional redundancy in members of the family Mimiviridae, suggesting this may be the major evolutionary force behind their giant genomes. One-Sentence SummaryFunctional redundancy preserves mimivirus genomic fiber and layer of fibrils formation.

microbiology↗

bioRxiv2022

Evolution of giant pandoravirus from small icosahedral viruses revealed by CRISPR/Cas9

Giant viruses (GVs) are a hotspot of unresolved controversies since their discovery, including the definition of "Virus" and the existence of a fourth domain of life1-3. While increasing knowledge of genome diversity has accumulated4, functional genomics was largely neglected. Here, we describe an experimental framework to genetically modify nuclear GVs and its host Acanthamoeba castellanii using CRISPR/Cas9, allowing us to uncover the evolution from small icosahedral viruses to amphora-shaped GVs. Ablation of the icosahedral major capsid protein in the evolutionary intermediate mollivirus highlights a stepwise transition in virion shape and size. We additionally demonstrate the existence of a reduced core essential genome in pandoravirus, reminiscent of their proposed smaller ancestors. Genetic expansion led to increased genome robustness, indicating selective pressures for adaptation to uncertain environments. Overall, we introduce new tools for manipulation of the unexplored genome of nuclear GVs and demonstrate that viral gigantism can arise as an emerging trait.

microbiology↗

bioRxiv2021

Toxoplasma gondii phosphatidylserine flippase complex ATP2B-CDC50.4 critically participates in microneme exocytosis

Regulated microneme secretion governs motility, host cell invasion and egress in the obligate intracellular apicomplexans. Intracellular calcium oscillations and phospholipid dynamics critically regulate micronemes exocytosis. Despite its importance for the lytic cycle of these parasites, molecular mechanistic details about exocytosis are still missing. Some members of the P4-ATPases act as flippases, changing the phospholipid distribution by translocation from the outer to the inner leaflet of the membrane. Here, the localization and function of the repertoire of P4-ATPases was investigated across the lytic cycle of Toxoplasma gondii. Of relevance, ATP2B and the non-catalytic subunit cell division control protein 50.4 (CDC50.4) form a stable heterocomplex at the parasite plasma membrane, essential for microneme exocytosis. This complex is responsible for flipping phosphatidylserine (PS), which presumably acts as a lipid mediator for the organelle fusion with the plasma membrane. DOC2.1, a previously described key egress and invasion factor, is shown here to be affected in its function in egress upon mutation on residues putatively involved in calcium binding. This study points toward the importance of PS in microneme exocytosis and unveils subtle differences in the signaling cascades leading to organelle secretion between intracellular and extracellular parasites to ensure egress and invasion, respectively. Author SummaryBiological membranes display diverse functions, including membrane fusion, which are conferred by a defined composition and organization of proteins and lipids. Apicomplexan parasites possess specialized secretory organelles (micronemes), implicated in motility, invasion and egress from host cells. Microneme exocytosis is already known to depends on phosphatidic acid for its fusion with the plasma membrane. Here we identify a type P4-ATPase and its CDC50 chaperone (ATP2B-CDC50.4) that act as flippase and contribute to the enrichment of phosphatidylserine (PS) in the inner leaflet of the parasite plasma membrane. PS and the previously described C2-containing protein DOC2.1 differentially participate in microneme exocytosis in the context of environmental changes. Overall, our results shed light on the importance membrane homeostasis and lipid composition in controlling microneme secretion.

cell biology↗