Unveiling the Zika virus: The ongoing Zika virus epidemic is of grave concern because of its apparent links to congenital microcephaly and Guillain-Barré syndrome. Sirohi et al. present a near-atomic-resolution structure of mature Zika virus determined by cryo-electron microscopy. The structure is mainly similar to that of other flaviviruses such as dengue virus; however, there are differences in a region that may be involved in binding to host receptors. The structure provides a foundation for analysis of the antigenicity and pathogenesis of Zika virus. Science , this issue p. 467

Many viruses go through a maturation step in the final stages of assembly before being transmitted to another host. The maturation process of flaviviruses is directed by the proteolytic cleavage of the precursor membrane protein (prM), turning inert virus into infectious particles. We have determined the 2.2 angstrom resolution crystal structure of a recombinant protein in which the dengue virus prM is linked to the envelope glycoprotein E. The structure represents the prM-E heterodimer and fits well into the cryo–electron microscopy density of immature virus at neutral pH. The pr peptide β-barrel structure covers the fusion loop in E, preventing fusion with host cell membranes. The structure provides a basis for identifying the stages of its pH-directed conformational metamorphosis during maturation, ending with release of pr when budding from the host.

Giant viruses such as Mimivirus isolated from amoeba found in aquatic habitats show biological sophistication comparable to that of simple cellular life forms and seem to evolve by similar mechanisms, including extensive gene duplication and horizontal gene transfer (HGT), possibly in part through a viral parasite, the virophage. We report here the isolation of “Marseille” virus, a previously uncharacterized giant virus of amoeba. The virions of Marseillevirus encompass a 368-kb genome, a minimum of 49 proteins, and some messenger RNAs. Phylogenetic analysis of core genes indicates that Marseillevirus is the prototype of a family of nucleocytoplasmic large DNA viruses (NCLDV) of eukaryotes. The genome repertoire of the virus is composed of typical NCLDV core genes and genes apparently obtained from eukaryotic hosts and their parasites or symbionts, both bacterial and viral. We propose that amoebae are “melting pots” of microbial evolution where diverse forms emerge, including giant viruses with complex gene repertoires of various origins.

Alphaviruses are significant animal and human pathogens — as demonstrated in recent outbreaks of infection with the mosquito-borne Chikungunya virus in India and southeast Asia. The E1 and E2 glycoproteins of alphaviruses are central to the way the virus infects host cells. The E1/E2 heterodimers that form spikes on the virus surface dissociate in the acidic conditions found in the internal vesicles of host cells, and E1 triggers infection by fusing with the endosomal membrane. Félix Rey and colleagues present the structure of Chikungunya virus envelope glycoprotein at neutral pH, and Michael Rossmann and colleagues reveal the structure of the envelope proteins of Sindbis virus at low pH. Sindbis virus can cause fever in humans and is the most extensively studied alphavirus. Comparison of the two structures provides insight into how fusion activation is controlled and points to possible vaccine targets. The E1 and E2 glycoproteins of alphaviruses form heterodimers and assemble into spikes on the virus surface, which mediate receptor binding and endocytosis. When the virion encounters acidic pH in the endosome E1 and E2 dissociate and E1 triggers fusion with the endosomal membrane. Two papers now provide the first crystal structures for glycoprotein complexes incorporating E2 at acidic and neutral pH, respectively. Together they provide insight into how fusion activation is controlled in alphaviruses. Alphaviruses are enveloped RNA viruses that have a diameter of about 700 Å and can be lethal human pathogens1. Entry of virus into host cells by endocytosis is controlled by two envelope glycoproteins, E1 and E2. The E2–E1 heterodimers form 80 trimeric spikes on the icosahedral virus surface1,2, 60 with quasi-three-fold symmetry and 20 coincident with the icosahedral three-fold axes arranged with T = 4 quasi-symmetry. The E1 glycoprotein has a hydrophobic fusion loop at one end and is responsible for membrane fusion3,4. The E2 protein is responsible for receptor binding5,6 and protects the fusion loop at neutral pH. The lower pH in the endosome induces the virions to undergo an irreversible conformational change in which E2 and E1 dissociate and E1 forms homotrimers, triggering fusion of the viral membrane with the endosomal membrane and then releasing the viral genome into the cytoplasm3,4. Here we report the structure of an alphavirus spike, crystallized at low pH, representing an intermediate in the fusion process and clarifying the maturation process. The trimer of E2–E1 in the crystal structure is similar to the spikes in the neutral pH virus except that the E2 middle region is disordered, exposing the fusion loop. The amino- and carboxy-terminal domains of E2 each form immunoglobulin-like folds, consistent with the receptor attachment properties of E2.

The tailed double-stranded DNA bacteriophages, or Caudovirales, constitute ~96% of all the known phages. Although these phages come in a great variety of sizes and morphology, their virions are mai...

Remarkable progress has been made during the past ten years in elucidating the structure of the bacteriophage T4 tail by a combination of three-dimensional image reconstruction from electron micrographs and X-ray crystallography of the components. Partial and complete structures of nine out of twenty tail structural proteins have been determined by X-ray crystallography and have been fitted into the 3D-reconstituted structure of the "extended" tail. The 3D structure of the "contracted" tail was also determined and interpreted in terms of component proteins. Given the pseudo-atomic tail structures both before and after contraction, it is now possible to understand the gross conformational change of the baseplate in terms of the change in the relative positions of the subunit proteins. These studies have explained how the conformational change of the baseplate and contraction of the tail are related to the tail's host cell recognition and membrane penetration function. On the other hand, the baseplate assembly process has been recently reexamined in detail in a precise system involving recombinant proteins (unlike the earlier studies with phage mutants). These experiments showed that the sequential association of the subunits of the baseplate wedge is based on the induced-fit upon association of each subunit. It was also found that, upon association of gp53 (gene product 53), the penultimate subunit of the wedge, six of the wedge intermediates spontaneously associate to form a baseplate-like structure in the absence of the central hub. Structure determination of the rest of the subunits and intermediate complexes and the assembly of the hub still require further study.