Oxidoreductase and Chaperone Activities Co-opted by the Nonenveloped Polyomaviruses during Entry.

Abstract

Nonenveloped viruses lack a lipid surface that would permit membrane fusion events and direct access to the cytosol of a host cell. Instead these viruses undergo conformational changes that enable them to bind to, disrupt, and penetrate a biological membrane leading to successful infection. This thesis work describes the mechanism for membrane penetration by polyomaviruses (Pys), a family of viruses that can cause several devastating pathologies in immunocompromised individuals. Pys are endocytosed and traffic to the endoplasmic reticulum (ER) where they must cross the ER membrane to cause infection. We find that multiple ER-resident proteins normally involved in protein folding, called PDI, ERp57 and ERp72 are important for infection of murine Py. In vitro assays reveal that a subset of PDI proteins act coordinately to disrupt capsid disulfide bonds and induce important conformational changes. This activation step drives ER membrane engagement where the virus can be recognized by additional cellular factors normally involved in retro-translocating misfolded ER proteins to the cytosol for degradation. Our research on another model Py, SV40, elucidates the function of two ER membrane proteins (DnaJB14 and DnaJB12) that facilitate this stage of membrane penetration. We demonstrate that DnaJB14 (B14) and DnaJB12 (B12) are part of a large protein complex that dynamically reorganizes into discrete foci within the ER membrane upon encountering SV40. Additionally, B14-B12 promote infection by recruiting multiple cytosolic chaperones to the site of membrane penetration. Specifically, we identify SGTA as one chaperone hijacked by SV40 for completing ER-to-cytosol transport. SGTA can physically engage SV40 in cells during entry and interact directly in vitro. Thus, this research supports a model whereby a nonenveloped virus co-opts several protein quality control systems for membrane penetration.