How non-enveloped viruses overcome host cell membranes is poorly understood. Here, we show that after endocytosis and transport to the endoplasmic reticulum (ER), but before crossing the ER membrane to the cytosol, incoming simian virus 40 particles are structurally remodelled leading to exposure of the amino-terminal sequence of the minor viral protein VP2. These hydrophobic sequences anchor the virus to membranes. A negatively charged residue, Glu 17, in the α-helical, membrane-embedded peptide is essential for infection, most likely by introducing an ‘irregularity’ recognized by the ER-associated degradation (ERAD) system for membrane proteins. Using a siRNA-mediated screen, the lumenal chaperone BiP and the ER-membrane protein BAP31 (both involved in ERAD) were identified as being essential for infection. They co-localized with the virus in discrete foci and promoted its ER-to-cytosol dislocation. Virus-like particles devoid of VP2 failed to cross the membrane. The results demonstrated that ERAD-factors assist virus transport across the ER membrane.
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We thank H. Ploegh, G. Shore, E. Wiertz, H. Meyer, H. Kasamatsu, A. Oppenheim, L. Hendershot and K. Johnsson for sharing reagents. We are grateful to A. Smith, P. Deprez, J. Kartenbeck and R. Mancini for preparing mPy and microsomes and for help with electron microscopy. We thank D. Roderer for help with circular dichroism spectroscopy measurements. We thank members of the Helenius group for discussions, especially M. Schelhaas. This work was supported by the National Science Foundation of Switzerland, the LipidX programme of SystemsX, the European research council (ERC) and ETH Zurich.
The authors declare no competing financial interests.
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Geiger, R., Andritschke, D., Friebe, S. et al. BAP31 and BiP are essential for dislocation of SV40 from the endoplasmic reticulum to the cytosol. Nat Cell Biol 13, 1305–1314 (2011). https://doi.org/10.1038/ncb2339
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