FEAR antiviral response pathway is independent of interferons and countered by poxvirus proteins

The human facilitates chromatin transcription (FACT) complex is a chromatin remodeller composed of human suppressor of Ty 16 homologue (hSpt16) and structure-specific recognition protein-1 subunits that regulates cellular gene expression. Whether FACT regulates host responses to infection remained unclear. We identify a FACT-mediated, interferon-independent, antiviral pathway that restricts poxvirus replication. Cell culture and bioinformatics approaches suggest that early viral gene expression triggers nuclear accumulation of SUMOylated hSpt16 subunits required for the expression of E26 transformation-specific sequence-1 (ETS-1)-a transcription factor that activates virus restriction programs. However, biochemical studies show that poxvirus-encoded A51R proteins block ETS-1 expression by outcompeting structure-specific recognition protein-1 binding to SUMOylated hSpt16 and by tethering SUMOylated hSpt16 to microtubules. Furthermore, A51R antagonism of FACT enhances poxvirus replication in human cells and virulence in mice. Finally, we show that FACT also restricts rhabdoviruses, flaviviruses and orthomyxoviruses, suggesting broad roles for FACT in antiviral immunity. Our study reveals the FACT-ETS-1 antiviral response (FEAR) pathway to be critical for eukaryotic antiviral immunity and describes a unique mechanism of viral immune evasion.

Poxvirus A51R proteins regulate microtubule stability and antagonize a cell-intrinsic antiviral response

Numerous viruses alter host microtubule (MT) networks during infection, but how and why they induce these changes is unclear in many cases. We show that the vaccinia virus (VV)-encoded A51R protein is a MT-associated protein (MAP) that directly binds MTs and stabilizes them by both promoting their growth and preventing their depolymerization. Furthermore, we demonstrate that A51R-MT interactions are conserved across A51R proteins from multiple poxvirus genera, and highly conserved, positively charged residues in A51R proteins mediate these interactions. Strikingly, we find that viruses encoding MT interaction-deficient A51R proteins fail to suppress a reactive oxygen species (ROS)-dependent antiviral response in macrophages that leads to a block in virion morphogenesis. Moreover, A51R-MT interactions are required for VV virulence in mice. Collectively, our data show that poxviral MAP-MT interactions overcome a cell-intrinsic antiviral ROS response in macrophages that would otherwise block virus morphogenesis and replication in animals.

Peripheral membrane proteins modulate stress tolerance by safeguarding cellulose synthases

Controlled primary cell wall remodeling allows plant growth under stressful conditions, but how these changes are conveyed to adjust cellulose synthesis is not understood. Here, we identify the TETRATRICOPEPTIDE THIOREDOXIN-LIKE (TTL) proteins as new members of the cellulose synthase complex (CSC) and describe their unique and hitherto unknown dynamic association with the CSC under cellulose-deficient conditions. We find that TTLs are essential for maintaining cellulose synthesis under high-salinity conditions, establishing a stress-resilient cortical microtubule array, and stabilizing CSCs at the plasma membrane. To fulfill these functions, TTLs interact with CELLULOSE SYNTHASE 1 (CESA1) and engage with cortical microtubules to promote their polymerization. We propose that TTLs function as bridges connecting stress perception with dynamic regulation of cellulose biosynthesis at the plasma membrane.

Pathogen-induced pH changes regulate the growth-defense balance in plants

Environmental adaptation of organisms relies on fast perception and response to external signals, which lead to developmental changes. Plant cell growth is strongly dependent on cell wall remodeling. However, little is known about cell wall‐related sensing of biotic stimuli and the downstream mechanisms that coordinate growth and defense responses. We generated genetically encoded pH sensors to determine absolute pH changes across the plasma membrane in response to biotic stress. A rapid apoplastic acidification by phosphorylation‐based proton pump activation in response to the fungus Fusarium oxysporum immediately reduced cellulose synthesis and cell growth and, furthermore, had a direct influence on the pathogenicity of the fungus. In addition, pH seems to influence cellulose structure. All these effects were dependent on the COMPANION OF CELLULOSE SYNTHASE proteins that are thus at the nexus of plant growth and defense. Hence, our discoveries show a remarkable connection between plant biomass production, immunity, and pH control, and advance our ability to investigate the plant growth‐defense balance.

The companion of cellulose synthase 1 confers salt tolerance through a Tau-like mechanism in plants

Microtubules are filamentous structures necessary for cell division, motility and morphology, with dynamics critically regulated by microtubule-associated proteins (MAPs). Here we outline the molecular mechanism by which the MAP, COMPANION OF CELLULOSE SYNTHASE1 (CC1), controls microtubule bundling and dynamics to sustain plant growth under salt stress. CC1 contains an intrinsically disordered N-terminus that links microtubules at evenly distributed points through four conserved hydrophobic regions. By NMR and live cell analyses we reveal that two neighboring residues in the first hydrophobic binding motif are crucial for the microtubule interaction. The microtubule-binding mechanism of CC1 is reminiscent to that of the prominent neuropathology-related protein Tau, indicating evolutionary convergence of MAP functions across animal and plant cells.

Cellulose-Microtubule Uncoupling Proteins Prevent Lateral Displacement of Microtubules during Cellulose Synthesis in Arabidopsis

Cellulose is the most abundant biopolymer on Earth and is the major contributor to plant morphogenesis. Cellulose is synthesized by plasma membrane-localized cellulose synthase complexes (CSCs). Nascent cellulose microfibrils become entangled in the cell wall, and further catalysis therefore drives the CSC forward through the membrane: a process guided by cortical microtubules via the protein CSI1/POM2. Still, it is unclear how the microtubules can withstand the forces generated by the motile CSCs to effectively direct CSC movement. Here, we identified a family of microtubule-associated proteins, the cellulose synthase-microtubule uncouplings (CMUs), that located as static puncta along cortical microtubules. Functional disruption of the CMUs caused lateral microtubule displacement and compromised microtubule-based guidance of CSC movement. CSCs that traversed the microtubules interacted with the microtubules via CSI1/POM2, which prompted the lateral microtubule displacement. Hence, we have revealed how microtubules can withstand the propulsion of the CSCs during cellulose biosynthesis and thus sustain anisotropic plant cell growth.