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BAP31 and BiP are essential for dislocation of SV40 from the endoplasmic reticulum to the cytosol


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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Figure 1: SV40 undergoes a structural change in the ER.
Figure 2: The N terminus of VP2 folds into an α-helix, integrates into the ER membrane and contains an essential residue, Glu 17.
Figure 3: SV40 infection depends on BAP31, BAP29 and RMA1.
Figure 4: Transport of SV40 to the ER is not affected following knockdown of BAP31.
Figure 5: BAP31, BAP29 and SV40 accumulate in discrete spots in the ER.
Figure 6: BAP31 interacts with the N-terminal peptide of VP2.
Figure 7: BiP is a critical factor in SV40 infection.
Figure 8: BAP31 and BiP are essential for the transport of SV40 to the cytosol.


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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.

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R.G. designed and carried out experiments and analysed the data. D.A., S.F., F.H. and S.L. carried out experiments. T.H. wrote the MATLAB program. R.G. and A.H. wrote the manuscript. A.H. supervised the work.

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Correspondence to Ari Helenius.

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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).

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