Chaillet, M. | Search bioRxiv

bioRxiv2023

Exploring the molecular composition of the multipass translocon in its native membrane environment

Multispanning membrane proteins are inserted into the endoplasmic reticulum membrane by the ribosome-bound multipass translocon machinery. Based on cryo-electron tomography and extensive subtomogram analysis, we reveal the composition and arrangement of multipass translocon components in their native membrane environment. The intramembrane chaperone complex PAT and the translocon associated protein (TRAP) complex associate substoichiometrically with the multipass translocon in a translation-dependent manner. While PAT is preferentially recruited to active complexes, TRAP primarily associates with inactive translocons. The subtomogram average of the TRAP-multipass translocon reveals intermolecular contacts between the luminal domains of TRAP and an unknown subunit of the BOS complex. AlphaFold modeling suggests this protein is NOMO, bridging the luminal domains of nicalin and TRAP. Collectively, our results visualize the interplay of accessory factors associated with multipass membrane protein biogenesis under near-native conditions.

cell biology↗

bioRxiv2023

Structure and replication cycle of a virus infecting climate-modulating alga Emiliania huxleyi

The globally distributed marine alga Emiliania huxleyi produces reflective calcite disks (coccoliths) that increase the albedo of ocean water and thus reduce the heat absorption in the ocean, which cools the Earths climate. The population density of E. huxleyi is restricted by nucleocytoplasmic large DNA viruses, including E. huxleyi virus 201 (EhV-201). Despite the impact of E. huxleyi viruses on the climate, there is limited information about their structure and replication. Here we show that the dsDNA genome inside the EhV-201 virion is protected by an inner membrane, capsid, and outer membrane decorated with numerous transmembrane proteins. The virions are prone to deformation, and parts of their capsids deviate from the icosahedral arrangement. EhV-201 virions infect E. huxleyi by using their fivefold vertex to bind to a host cell and fuse the viruss inner membrane with the plasma membrane. Whereas the replication of EhV-201 probably occurs in the nucleus, virions assemble in the cytoplasm at the surface of endoplasmic reticulum-derived membrane segments. Genome packaging initiates synchronously with the capsid assembly and completes through an aperture in the forming capsid. Upon the completion of genome packaging, the capsids change conformation, which enables them to acquire an outer membrane by budding into intracellular vesicles. EhV-201 infection induces a loss of surface protective layers from E. huxleyi cells, which allows the continuous release of virions by exocytosis. Our results provide insight into how EhVs bypass the surface protective layers of E. huxleyi and exploit the organelles of an infected cell for progeny assembly.

molecular biology↗