Dengue virus (DENV) is a significant human pathogen that causes millions of infections and results in about 24,000 deaths each year. Dendritic cell-specific ICAM3 grabbing nonintegrin (DC-SIGN), abundant in immature dendritic cells, was previously reported as being an ancillary receptor interacting with the surface of DENV. The structure of DENV in complex with the carbohydrate recognition domain (CRD) of DC-SIGN was determined by cryo-electron microscopy at 25 A resolution. One CRD monomer was found to bind to two glycosylation sites at Asn67 of two neighboring glycoproteins in each icosahedral asymmetric unit, leaving the third Asn67 residue vacant. The vacancy at the third Asn67 site is a result of the nonequivalence of the glycoprotein environments, leaving space for the primary receptor binding to domain III of E. The use of carbohydrate moieties for receptor binding sites suggests a mechanism for avoiding immune surveillance.

Many pleomorphic, lipid-enveloped viruses encode matrix proteins that direct their assembly and budding, but the mechanism of this process is unclear. We have combined X-ray crystallography and cryoelectron tomography to show that the matrix protein of Newcastle disease virus, a paramyxovirus and relative of measles virus, forms dimers that assemble into pseudotetrameric arrays that generate the membrane curvature necessary for virus budding. We show that the glycoproteins are anchored in the gaps between the matrix proteins and that the helical nucleocapsids are associated in register with the matrix arrays. About 90% of virions lack matrix arrays, suggesting that, in agreement with previous biological observations, the matrix protein needs to dissociate from the viral membrane during maturation, as is required for fusion and release of the nucleocapsid into the host's cytoplasm. Structure and sequence conservation imply that other paramyxovirus matrix proteins function similarly.

Cryo-electron microscopy three-dimensional reconstructions have been made of mature and of emptied bacteriophage phi29 particles without making symmetry assumptions. Comparisons of these structures with each other and with the phi29 prohead indicate how conformational changes might initiate successive steps of assembly and infection. The 12 adsorption capable 'appendages' were found to have a structure homologous to the bacteriophage P22 tailspikes. Two of the appendages are extended radially outwards, away from the long axis of the virus, whereas the others are around and parallel to the phage axis. The appendage orientations are correlated with the symmetry-mismatched positions of the five-fold related head fibers, suggesting a mechanism for partial cell wall digestion upon rotation of the head about the tail when initiating infection. The narrow end of the head-tail connector is expanded in the mature virus. Gene product 3, bound to the 5' ends of the genome, appears to be positioned within the expanded connector, which may potentiate the release of DNA-packaging machine components, creating a binding site for attachment of the tail.

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