A Cytosolic Chaperone Complexes with Dynamic Membrane J-Proteins and Mobilizes a Nonenveloped Virus out of the Endoplasmic Reticulum

Abstract

Nonenveloped viruses undergo conformational changes that enable them to bind to, disrupt, and penetrate a biological membrane leading to successful infection. We assessed whether cytosolic factors play any role in the endoplasmic reticulum (ER) membrane penetration of the nonenveloped SV40. We find the cytosolic SGTA-Hsc70 complex interacts with the ER transmembrane J-proteins DnaJB14 (B14) and DnaJB12 (B12), two cellular factors previously implicated in SV40 infection. SGTA binds directly to SV40 and completes ER membrane penetration. During ER-to-cytosol transport of SV40, SGTA disengages from B14 and B12. Concomitant with this, SV40 triggers B14 and B12 to reorganize into discrete foci within the ER membrane. B14 must retain its ability to form foci and interact with SGTA-Hsc70 to promote SV40 infection. Our results identify a novel role for a cytosolic chaperone in the membrane penetration of a nonenveloped virus and raise the possibility that the SV40-induced foci represent cytosol entry sites.

Author Summary

The nonenveloped simian virus 40 (SV40) is a model member of the Polyomaviridae family of viruses containing several related species that cause diseases in immunocompromised individuals. As with other nonenveloped viruses, the membrane penetration step during SV40 entry is mechanistically obscure. Productive SV40 infection requires trafficking of the viral particle to the endoplasmic reticulum (ER) from where it penetrates the ER membrane to reach the cytosol; further transport of the virus into the nucleus causes infection. How SV40 crosses the ER membrane is an enigmatic step. Here, we identify a cytosolic chaperone protein that physically engages SV40 and facilitates virus ER-to-cytosol transport. This factor called SGTA is hijacked specifically at the site of membrane penetration due to its recruitment by ER membrane proteins B14 and B12 previously implicated in supporting virus infection. Additionally, we observe that B14 and B12 reorganize during SV40 entry into discrete foci on the ER membrane. These virus-induced structures likely represent exit sites for the viral particles and could serve to transiently recruit high concentrations of SGTA to complete membrane penetration. Our data reveal that a cytosolic chaperone can play a direct role in membrane penetration of a nonenveloped virus.