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Caveolae have long been considered to be an alternative endocytic pathway, with distinct cargoes, but generally similar functions, to clathrin-coated pits. Here we suggest that the mechanisms of caveola formation and their scission are tightly interlinked and rely on specific lipids. These mechanisms are fundamentally different to those driving the formation and fission of coated pits. Both formation and scission of caveolae are driven by lipid-induced shaping of the caveolar domain, and we present biophysical models for lipid-driven curvature generation and its coupling with scission. In addition, we propose that these new insights have important implications for understanding the function of endocytosis mediated by caveolae. Rather than a parallel endocytic pathway for protein cargo, we argue that caveolae are a lipid-sensitive mobilized multifunctional surface domain.

Flaviviruses replicate their genomes in replication organelles (ROs) formed as bud-like invaginations on the endoplasmic reticulum membrane, which also functions as the site for virion assembly. While this localization is well established, it is not known to what extent viral membrane remodeling, genome replication, virion assembly, and maturation are coordinated. Here, we image tick-borne flavivirus replication in human cells using cryo-electron tomography. We find that the RO membrane bud is shaped by a combination of a curvature-establishing membrane modification and the pressure from intraluminal template RNA. A protein complex at the RO base extends to an adjacent membrane, where immature virus particles bud. Naturally occurring furin site variants determine whether virus particles mature in the immediate vicinity of ROs. We further visualize replication in mouse brain tissue by cryo-electron tomography. Taken together, these findings reveal a close spatial coupling of flavivirus genome replication, budding, and maturation.

The nuclear pore complex (NPC) is composed of multiple nucleoporins (NUPs) and enables the exchange of RNA and proteins between the nucleus and cytoplasm. NUP98 is one of the major components of the NPC, being involved in the RNA export pathway by interacting with several transport factors. Previous studies have suggested both proviral and antiviral functions of NUP98 in viral infection, yet little is known about its function in orthoflavivirus infection. In this study we show that NUP98 is a proviral cellular protein that is recruited to the cytoplasm during orthoflavivirus infection. We observe that NUP98 is found specifically in the vicinity of the replication vesicles during infections with tick-borne encephalitis virus, Japanese encephalitis virus, and yellow fever virus. Furthermore, using surface plasmon resonance, cross-link immunoprecipitation, and cross-link immunoprecipitation-sequencing we observe that the C-Terminal domain of NUP98 directly interacts with a conserved site of the viral RNA (vRNA) in the E coding region promoting viral replication. We identified a peptide that binds to NUP98 that is antivirally active against several orthoflaviviruses by outcompeting the binding between NUP98 and vRNA, making NUP98 an attractive target for antiviral development.

Orthoflaviviruses are RNA viruses that cause serious diseases in humans, with currently no antivirals available. Targeting host factors is emerging as an attractive antiviral approach. However, as a first step, there is a need to understand which host proteins are hijacked and for what purpose. Here, using a combination of fluorescence microscopy, knock-down, crosslinking immunoprecipitation sequencing, mass spectrometry, and in vitro and biophysical assays, we identify nucleoporin-153 (NUP153) as a proviral factor during orthoflavivirus infection. We show that NUP153 is recruited to the virus amplification site on the endoplasmic reticulum to impact the structural to nonstructural viral protein ratios. We find that NUP153 interacts with both the viral proteins NS3 and NS5, and a highly conserved G-rich motif on the viral RNA. These interactions specifically promote the production of viral structural proteins, leading to an efficient virion assembly, virus release and spread to new cells. We propose that NUP153 acts as a key regulator in viral protein ratios, a mechanism that appears conserved among orthoflaviviruses.

Caveolae are specialized plasma membrane domains with a unique lipid composition. Lipid peroxidation has recently been implicated in triggering caveola disassembly, releasing cavin proteins to regulate oxidative-stress-associated cellular processes, particularly ferroptosis. Here, we investigated how specific lipids influence caveola formation and their response to oxidative stress. A targeted screening of pro-ferroptotic enzymes identified ACSL4, a key enzyme in synthesizing polyunsaturated fatty acid (PUFA)-linked phospholipids, and ether phospholipid biosynthesis enzymes as critical regulators of caveola formation. Membrane-incorporated omega-6 PUFAs promoted caveola formation, while their displacement by omega-3 PUFAs or monounsaturated fatty acids disrupted this process. Importantly, oxidation of omega-6 PUFA chains in phosphatidylethanolamine (PE) triggered caveola disassembly during lipid peroxidation, potentially by affecting cavin-membrane interactions. These findings unveil a new model for caveola formation and signaling, linking caveola dynamics to ferroptosis with pro-ferroptotic lipids as essential caveolar components and key control points for caveola disassembly under oxidative stress.

Flavivirus infection involves extensive remodeling of the endoplasmic reticulum (ER), which is key to both the replication of the viral RNA genome as well as the assembly and release of new virions. However, little is known about how viral proteins and host factors cooperatively facilitate such a vast transformation of the ER, and how this influences the different steps of the viral life cycle. In this study, we screened for host proteins that were enriched in close proximity to the tick-borne encephalitis virus (TBEV) protein NS4B and found that the top candidates were coupled to trafficking between ER exit sites (ERES) and the Golgi. We characterized the role of ACBD3, one of the identified proteins, and showed that it promotes TBEV infection. Depletion of ACBD3 inhibited virus replication and resulted in abnormal transformation of the ER, leading to reduced virion release. ACBD3's proviral mechanism did not involve the recruitment of PI4PK as previously described for enteroviruses. Instead, productive TBEV infection required the full-length ACBD3, which localizes to ER-Golgi contact sites together with NS4B. We propose that NS4B and ACBD3 promote replication by coordinating the transformation of the ER, which is required for RNA replication and particle release. The transformation involves direct coupling to the Golgi which facilitates efficient virion transport.

IMPORTANCE: Flaviviruses like tick-borne encephalitis have significant effects on human health. During flavivirus infection, the viral particles enter the host cells and transform the endoplasmic reticulum (ER), which is a membranous organelle and the main site of cellular protein synthesis. Although this is critical for successful infection, the details of the process are unknown. Here, we found that the viral protein NS4B and the host protein ACBD facilitate this transformation by ensuring that the ER is coupled to the Golgi apparatus, the organelle responsible for transporting material out of the cell. TBEV uses ACBD3 to guarantee that the connection sites between the transformed ER and the Golgi remain functional so that RNA is replicated and the produced viral particles are exported from the cell and can infect further cells. Our work sheds light both on the basic biology of flavivirus infection, and virus-induced remodeling of membranous organelles.

The large GTPase dynamin has a crucial role in endocytosis, working at the neck of clathrin-coated pits to drive vesicular scission. Until recently, dynamin was believed to regulate endocytosis through caveolae in a similar fashion. However, recent work calls for a serious reassessment of the role of dynamin in endocytosis by caveolae.

Caveolae are atypical plasma membrane invaginations that take part in lipid sorting and regulation of oxidative and mechanical plasma membrane stress. Caveola formation requires caveolin, cavin, and specific lipid types. The recent advances in understanding the structure and assembly of caveolin and cavin complexes within the membrane context have clarified the fundamental processes underlying caveola biogenesis. In addition, the curvature of the caveola membrane is controlled by the regulatory proteins EHD2, pacsin2, and dynamin2, which also function to restrain the scission of caveolae from the plasma membrane (PM). Here, this is integrated with novel insights on caveolae as lipid and mechanosensing complexes that can dynamically flatten or disassemble to counteract mechanical, and oxidative stress.

Caveolae are small membrane invaginations that generally are stably attached to the plasma membrane. Their release is believed to depend on the GTPase dynamin 2 (Dyn2), in analogy with its role in fission of clathrin-coated vesicles. The mechanistic understanding of caveola fission is, however, sparse. Here, we used microscopy-based tracking of individual caveolae in living cells to determine the role of Dyn2 in caveola dynamics. We report that Dyn2 stably associated with the bulb of a subset of caveolae, but was not required for formation or fission of caveolae. Dyn2-positive caveolae displayed longer plasma membrane duration times, whereas depletion of Dyn2 resulted in shorter duration times and increased caveola fission. The stabilizing role of Dyn2 was independent of its GTPase activity and the caveola stabilizing protein EHD2. Thus, we propose that, in contrast to the current view, Dyn2 is not a core component of the caveolae machinery, but rather functions as an accessory protein that restrains caveola internalization.

Tick-borne encephalitis virus is an enveloped, pathogenic, RNA virus in the family Flaviviridae, genus Flavivirus. Viral particles are formed when the nucleocapsid, consisting of an RNA genome and multiple copies of the capsid protein, buds through the endoplasmic reticulum membrane and acquires the viral envelope and the associated proteins. The coordination of the nucleocapsid components to the sites of assembly and budding are poorly understood. Here, we investigate the interactions of the wild-type and truncated capsid proteins with membranes with biophysical methods and model membrane systems. We show that capsid protein initially binds membranes via electrostatic interactions with negatively-charged lipids, which is followed by membrane insertion. Additionally, we show that membrane-bound capsid protein can recruit viral genomic RNA. We confirm the biological relevance of the biophysical findings by using mass spectrometry to show that purified virions contain negatively-charged lipids. Our results suggest that nucleocapsid assembly is coordinated by negatively-charged membrane patches on the endoplasmic reticulum and that the capsid protein mediates direct contacts between the nucleocapsid and the membrane.