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Interactions of influenza A virus (IAV) with sialic acid (SIA) receptors determine viral fitness and host tropism. Binding to mucus decoy receptors and receptors on epithelial host cells is determined by a receptor-binding hemagglutinin (HA), a receptor-destroying neuraminidase (NA) and a complex in vivo receptor-repertoire. The crucial but poorly understood dynamics of these multivalent virus-receptor interactions cannot be properly analyzed using equilibrium binding models and endpoint binding assays. In this study, the use of biolayer interferometric analysis revealed the virtually irreversible nature of IAV binding to surfaces coated with synthetic sialosides or engineered sialoglycoproteins in the absence of NA activity. In addition to HA, NA was shown to be able to contribute to the initial binding rate while catalytically active. Virus-receptor binding in turn contributed to receptor cleavage by NA. Multiple low-affinity HA-SIA interactions resulted in overall extremely high avidity but also permitted a dynamic binding mode, in which NA activity was driving rolling of virus particles over the receptor-surface. Virus dissociation only took place after receptor density of the complete receptor-surface was sufficiently decreased due to NA activity of rolling IAV particles. The results indicate that in vivo IAV particles, after landing on the mucus layer, reside continuously in a receptor-bound state while rolling through the mucus layer and over epithelial cell surfaces driven by the HA-NA-receptor balance. Quantitative BLI analysis enabled functional examination of this balance which governs this dynamic and motile interaction that is expected to be crucial for penetration of the mucus layer and subsequent infection of cells by IAV but likely also by other enveloped viruses carrying a receptor-destroying enzyme in addition to a receptor-binding protein. [ABSTRACT FROM AUTHOR]

To understand how chromatin domains coordinate gene expression, we dissected select genetic elements organizing topology and transcription around the Prdm14 super enhancer in mouse embryonic stem cells. Taking advantage of allelic polymorphisms, we developed methods to sensitively analyze changes in chromatin topology, gene expression, and protein recruitment. We show that enhancer insulation does not rely strictly on loop formation between its flanking boundaries, that the enhancer activates the Slco5a1 gene beyond its prominent domain boundary, and that it recruits cohesin for loop extrusion. Upon boundary inversion, we find that oppositely oriented CTCF terminates extrusion trajectories but does not stall cohesin, while deleted or mutated CTCF sites allow cohesin to extend its trajectory. Enhancer-mediated gene activation occurs independent of paused loop extrusion near the gene promoter. We expand upon the loop extrusion model to propose that cohesin loading and extrusion trajectories originating at an enhancer contribute to gene activation. [Display omitted] • An enhancer initiates loop extrusion trajectories that delineate activatable genes • Enhancer insulation does not depend strictly on looping between its CTCF boundaries • A loop-engaged inverted CTCF boundary can also block cohesin extrusion trajectories • A strong CTCF boundary permits rare but productive enhancer-promoter contacts The three-dimensional structure of the genome, organized by architectural proteins CTCF and cohesin, has a strong influence on how genes are expressed. Here, Vos et al. allele-specifically dissect an active genomic region to reveal how an enhancer and CTCF boundaries organize cohesin extrusion trajectories, build structural domains, and regulate genes. [ABSTRACT FROM AUTHOR]