In many bacterial viruses and in certain animal viruses, the doublestranded DNA genome enters and exits the capsid through a portal gatekeeper. We report a pseudoatomic structure of a complete portal system. The bacteriophage SPP1 gatekeeper is composed of dodecamers of the portal protein gp6, the adaptor gp15, and the stopper gp16. The solution structures of gp15 and gp16 were determined by NMR. They were then docked together with the X-ray structure of gp6 into the electron density of the Ϸ1-MDa SPP1 portal complex purified from DNA-filled capsids. The resulting structure reveals that gatekeeper assembly is accompanied by a large rearrangement of the gp15 structure and by folding of a flexible loop of gp16 to form an intersubunit parallel ␤-sheet that closes the portal channel. This stopper system prevents release of packaged DNA. Disulfide cross-linking between ␤-strands of the stopper blocks the key conformational changes that control genome ejection from the virus at the beginning of host infection.NMR ͉ structure docking ͉ unstructured proteins ͉ virus assembly ͉ virus infection

International audienc

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Secondary electron image created by Gianluca Cattinari and colleagues showing the interaction between Natural Rubber (NR) globules and CarbonBlack (CB) aggregates occurring in the liquid state. Samples have been in liquid condition using vapour of osmium, preventing the immersion in a liquid solution containing the fixative agent, and allowing exploitation of both secondary electron (SE) and backscattered electron (BSE) imaging modes. This protocol allowed us to image the nanoscale between CB filler and NR globules highlighting a formation of the latter. DOI: https://doi.org/10.1002/app.50221

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