RELION: implementation of a Bayesian approach to cryo-EM structure determination

Orthobunyavirus ultrastructure and the curious tripodal glycoprotein spike

The genus Orthobunyavirus within the family Bunyaviridae constitutes an expanding group of emerging viruses, which threaten human and animal health. Despite the medical importance, little is known about orthobunyavirus structure, a prerequisite for understanding virus assembly and entry. Here, using electron cryo-tomography, we report the ultrastructure of Bunyamwera virus, the prototypic member of this genus. Whilst Bunyamwera virions are pleomorphic in shape, they display a locally ordered lattice of glycoprotein spikes. Each spike protrudes 18 nm from the viral membrane and becomes disordered upon introduction to an acidic environment. Using sub-tomogram averaging, we derived a three-dimensional model of the trimeric pre-fusion glycoprotein spike to 3-nm resolution. The glycoprotein spike consists mainly of the putative class-II fusion glycoprotein and exhibits a unique tripod-like arrangement. Protein–protein contacts between neighbouring spikes occur at membrane-proximal regions and intra-spike contacts at membrane-distal regions. This trimeric assembly deviates from previously observed fusion glycoprotein arrangements, suggesting a greater than anticipated repertoire of viral fusion glycoprotein oligomerization. Our study provides evidence of a pH-dependent conformational change that occurs during orthobunyaviral entry into host cells and a blueprint for the structure of this group of emerging pathogens.

Orthobunyaviruses comprise a group of emerging arboviruses within the Bunyaviridae, the largest family of membrane-containing viruses. In spite of the continued medical impact upon human and animal health, little is known about orthobunyavirus structure or the process of host cell entry. Here, we address this paucity of information through electron cryo-microscopy analysis of Bunyamwera virus, the prototypic representative of this genus. We reveal that Bunyamwera virions are pleomorphic and display locally-ordered lattices of viral glycoprotein spikes on the envelope surface. The three-dimensional structure of the glycoprotein spike was resolved to 3.0-nm resolution. The spike is composed of the attachment and fusion glycoproteins and comprises a unique tripodal organization. This glycoprotein arrangement contrasts those observed in other virus families. Consistent with the established pH-dependent mechanism of membrane fusion during host cell entry, we provide evidence for the disruption of this tripodal assembly upon exposure to acidic environments. These data constitute a blueprint for orthobunyavirus architecture and support a case for broadened structural diversity within the Bunyaviridae family.

Ribosome structures to near-atomic resolution from thirty thousand cryo-EM particles

Although electron cryo-microscopy (cryo-EM) single-particle analysis has become an important tool for structural biology of large and flexible macro-molecular assemblies, the technique has not yet reached its full potential. Besides fundamental limits imposed by radiation damage, poor detectors and beam-induced sample movement have been shown to degrade attainable resolutions. A new generation of direct electron detectors may ameliorate both effects. Apart from exhibiting improved signal-to-noise performance, these cameras are also fast enough to follow particle movements during electron irradiation. Here, we assess the potentials of this technology for cryo-EM structure determination. Using a newly developed statistical movie processing approach to compensate for beam-induced movement, we show that ribosome reconstructions with unprecedented resolutions may be calculated from almost two orders of magnitude fewer particles than used previously. Therefore, this methodology may expand the scope of high-resolution cryo-EM to a broad range of biological specimens.DOI:http://dx.doi.org/10.7554/eLife.00461.001.