Missense mutations in MORC2 cause neuropathies including spinal muscular atrophy and Charcot–Marie–Tooth disease. We recently identified MORC2 as an effector of epigenetic silencing by the human silencing hub (HUSH). Here we report the biochemical and cellular activities of MORC2 variants, alongside crystal structures of wild-type and neuropathic forms of a human MORC2 fragment comprising the GHKL-type ATPase module and CW-type zinc finger. This fragment dimerizes upon binding ATP and contains a hinged, functionally critical coiled-coil insertion absent in other GHKL ATPases. We find that dimerization and DNA binding of the MORC2 ATPase module transduce HUSH-dependent silencing. Disease mutations change the dynamics of dimerization by distinct structural mechanisms: destabilizing the ATPase-CW module, trapping the ATP lid, or perturbing the dimer interface. These defects lead to the modulation of HUSH function, thus providing a molecular basis for understanding MORC2-associated neuropathies.

Endogenous viral elements (EVEs), accounting for 15% of our genome, serve as a genetic reservoir from which new genes can emerge. Nematode EVEs are particularly diverse and informative of virus evolution. We identify Atlas virus—an intact retrovirus-like EVE in the human hookworm Ancylostoma ceylanicum , with an envelope protein genetically related to G N -G C glycoproteins from the family Phenuiviridae. A cryo-EM structure of Atlas G C reveals a class II viral membrane fusion protein fold not previously seen in retroviruses. Atlas G C has the structural hallmarks of an active fusogen. Atlas G C trimers insert into membranes with endosomal lipid compositions and low pH. When expressed on the plasma membrane, Atlas G C has cell-cell fusion activity. With its preserved biological activities, Atlas G C has the potential to acquire a cellular function. Our work reveals structural plasticity in reverse-transcribing RNA viruses.

Endogenous viral elements (EVEs), accounting for 15% of our genome, serve as a genetic reservoir from which new genes can emerge. Nematode EVEs are particularly diverse and informative of virus evolution. We identify Atlas virus – an intact retrovirus-like EVE in the human hookworm Ancylostoma ceylanicum, with an envelope protein genetically related to GN- GC glycoproteins from phleboviruses. A cryo-EM structure of Atlas GC reveals a class II viral membrane fusion protein fold not previously seen in retroviruses. Atlas GC has the structural hallmarks of an active fusogen. Atlas GC trimers insert into membranes with endosomal lipid compositions and low pH. When expressed on the plasma membrane, Atlas GC has cell-cell fusion activity. RNA-Seq data analysis detected transcripts mapping to Atlas virus at different stages of hookworm development. With its preserved biological activities, Atlas GC has the potential to acquire a cellular function. Our work reveals structural plasticity in reverse-transcribing RNA viruses.

Inflammasomes induce cell death in response to infection, chemical entities and cell damage. Human genetics and animal models have identified the NLRP3 inflammasome as a gatekeeper of caspase-1-dependent pyroptosis. NLRP3 activation induces polymerization of ASC into a single, micron-scale perinuclear punctum, where caspase-1 is activated. These puncta have yet to be imaged at sufficient resolution to resolve their ultrastructure. Here, we apply correlative cryo-light microscopy and cryo-electron tomography to visualize ASC/caspase-1 puncta and mitochondrial membrane remodelling within NLRP3-activated cells. The puncta are composed primarily of branched ASC filaments. The N-terminal pyrin domain forms an 8-10 nm-diameter tubular core, which is decorated by the flexibly linked C-terminal caspase recruitment domain. The variable filament density allows ribosomes and trans-Golgi-like vesicles to permeate the network. We propose this provides structural integrity while allowing macromolecules and vesicles to diffuse in or bind with the necessary density, timing and localization.

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