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Icosahedral double-stranded DNA viruses use a single portal for genome delivery and packaging. The extensive structural similarity revealed by such portals in diverse viruses, as well as their invariable positioning at a unique icosahedral vertex, led to the consensus that a particular, highly conserved vertex-portal architecture is essential for viral DNA translocations. Here we present an exception to this paradigm by demonstrating that genome delivery and packaging in the virus Acanthamoeba polyphaga mimivirus occur through two distinct portals. By using high-resolution techniques, including electron tomography and cryo-scanning electron microscopy, we show that Mimivirus genome delivery entails a large-scale conformational change of the capsid, whereby five icosahedral faces open up. This opening, which occurs at a unique vertex of the capsid that we coined the “stargate”, allows for the formation of a massive membrane conduit through which the viral DNA is released. A transient aperture centered at an icosahedral face distal to the DNA delivery site acts as a non-vertex DNA packaging portal. In conjunction with comparative genomic studies, our observations imply a viral packaging pathway akin to bacterial DNA segregation, which might be shared by diverse internal membrane–containing viruses.

The integral endoplasmic reticulum (ER)-membrane protein VAP-B interacts with various lipid-transfer/binding proteins containing an FFAT motif through its N-terminal MSP domain. A genetic mutation within its MSP domain, P56S, was identified in familial forms of motor neuron diseases. This mutation induces the formation of insoluble VAP-B(P56S) protein aggregates by an unknown mechanism. In this study, we defined the structural requirements for VAP-B oligomerization and demonstrated their contribution for VAP-B(P56S) aggregation and neurotoxicity. We show that the oligomerization of VAP-B is mainly mediated by its coiled-coil domain and that the GXXXG dimerization motif within the transmembrane domain mediates transmembrane domains self-association but is insufficient to drive VAP-B oligomerization. We further show that the oligomerization of the wild-type VAP-B is independent of its MSP domain. However, we found that the P56S mutation induces conformational changes within the MSP domain and facilitates its propensity to aggregate by exposing hydrophobic patches to the solvent. These conformational changes have no direct effect on FFAT binding. Rather, they enhance VAP-B(P56S) oligomerization driven by the combined contributions of the coiled-coil and the transmembrane domains, thereby preventing accessibility to FFAT-binding site, facilitating the production of VAP-B(P56S)-insoluble aggregates and consequently its neurotoxicity. These results shed light on the mechanism by which VAP-B(P56S) aggregates are formed and induce familial motor neuron diseases.Protein aggregation and inclusion body formation are common pathological features of many neurological disorders including Huntington disease, Alzheimer disease, Parkinson disease, and amyotrophic lateral sclerosis (ALS) 3 (1, 2).ALS is a fatal neurodegenerative disorder involving progressive loss of motor neurons in the cerebral cortex, brain stem, and spinal cord (3). Approximately 10% of the cases are familial, of which 20% are caused by dominantly inherited mutations in the Cu/Zn-superoxide dismutase (SOD) gene (4, 5). Recently, mutations in other genes, including the human VAP-B, have been identified in ALS patients (6 -8).The mutation in VAP-B substitutes a highly conserved proline residue at position 56 by a serine (P56S) and is associated with three forms of human motor neuron diseases: a late-onset spinal muscular atrophy, an atypical ALS type 8, and a typical severe ALS with rapid progression (7, 9). Atypical ALS type 8 is an autosomal dominant slowly progressive disorder characterized by fasciculation, cramps, and postural tremor. It is currently unknown how this P56S mutation induces motor neuron degeneration. Nevertheless, recent studies suggest that the VAP-B(P56S) mutant acts in a dominant fashion through dimerization with the wild-type protein and inactivating the heterodimer and that expression of the VAP-B(P56S) mutant induces the formation of insoluble protein aggregates (7, 10 -15). Indeed, co-expression studies suggest that wild-type VAP-B is...

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