PLoS Pathog. 11, e1005086 (2015)

Credit: PLOS PATHOG.

The non-enveloped polyomavirus SV40 enters host cells via the secretory pathway, where it gains access to the ER and undergoes remodeling that makes the virus competent to integrate into the ER membrane. From there, the virus is extracted into the cytosol, where it can then travel to the nucleus to continue its path towards establishing infection. Several host proteins are known to be involved in ER-to-cytosol translocation of SV40, including an ER membrane–localized complex consisting of heat shock protein Hsc70, the ATPase-stimulating J protein called B14 and the co-chaperone SGTA. Searching to characterize the role of this complex in ER-to-cytosol translocation of SV40, Ravindran et al. used an immunopurification strategy to find an additional binding partner in the complex, Hsp105, whose recruitment to the complex via B14 is increased during viral infection. An RNAi approach and mutational analysis showed that Hsp105's nucleotide exchange activity is important for infection by SV40. In addition, the authors found that Hsp105 is important in promoting cytosolic arrival of SV40 in a permeabilized cytosol arrival assay. They corroborated this finding with an imaging approach that also characterized the formation of SV40-induced ER membrane foci, which could be enhanced by depleting Hsp105 or decreased by overexpressing Hsp105, suggesting that the foci represent the cytosol entry site for the virus. Hsp105 could interact with SV40 in vitro as well as in immunoprecipitation experiments. The SV40-Hsp105 interaction is detectable at approximately the same time as virus enters the cytosol, suggesting that the interaction takes place at the ER-cytosol interface. Indeed, membrane-associated Hsp105 binds to SV40. Sucrose gradient sedimentation assays were consistent with a mechanism whereby the Hsp105–B14–Hsc70 complex (but not the individual proteins) disassembles the virus for cytosolic entry. The authors proposed that an iterative binding-release of SV40 by Hsp105–Hsc70 initiates disassembly of membrane-embedded virus to facilitate the ER extraction process, in a mechanism that is reminiscent of ER-to-cytosol translocation of misfolded proteins called ERAD.