To infect cells, SV40 VP1 binds to the glycolipid ganglioside GM1 on the host cell surface, inducing membrane tubulation that initiates internalization. And third, calciums bound to the virus clamp together different pentamers to increase capsid stabilization. Second, the VP1 C-terminus invades a neighboring VP1 pentamer to provide inter-pentamer support. First, disulfide bonds present throughout the virus stabilize it. Three additional forces support the overall viral architecture. VP1 also binds to the ∼5 kb viral DNA genome buried within the virus through electrostatic interactions. Each VP1 pentamer engages the internal proteins VP2 and VP3 through hydrophobic interactions. Structurally, SV40 is composed of 72 pentamers of the VP1 coat assembled into an icosahedral viral capsid. Here we address SV40's membrane transport process. Important questions include: what reaction sequence initiates membrane penetration? What is the nature of the viral conformational change and identity of the membrane penetrating species? What viral and host components control the penetration process, and how is membrane transport achieved? As it is unknown whether this scenario reflects the pathway in cells, establishing a cell-based assay that monitors non-enveloped virus membrane penetration affords the opportunity to study this event's physiological mechanism. They then engage the limiting membrane to disrupt its integrity, enabling the virus to cross the membrane. In this model, the virus undergoes conformational changes by interacting with host factors, culminating in the formation of a hydrophobic viral particle or release of a lytic peptide. However, a general model describing how they breach this membrane based largely on in vitro studies is emerging. The mechanism by which non-enveloped viruses such as simian virus 40 (SV40) and the murine polyomavirus (mPy) penetrate the host cell's membrane to cause infection is enigmatic. Together, our data illuminate the cellular mechanism by which a non-enveloped virus penetrates the limiting membrane of a target cell during infection. In addition to this finding, we also pinpoint viral and host components that control the ER-to-cytosol membrane transport event. This result suggests that the ER membrane can accommodate translocation of a large protein complex, possibly through either a sizeable protein channel or the ER membrane bilayer. Strikingly, we uncovered SV40 breaches the ER membrane as a large and intact viral particle, despite the conformational changes it experiences in the ER lumen. Here we established a cell-based assay to elucidate the molecular mechanism by which the non-enveloped SV40 penetrates the endoplasmic reticulum (ER) membrane to access the cytosol, a critical step in infection. Indeed, most available insights on membrane transport of non-enveloped viruses are built upon in vitro studies. While the mechanism by which an enveloped virus breaches the limiting membrane of a host cell is well-characterized, this membrane penetration process is poorly understood for non-enveloped viruses. They also suggest that the ER membrane supports passage of a large particle, potentially through either a sizeable protein-conducting channel or the lipid bilayer.īiological membranes represent a major barrier during viral infection. Our results identify the sequence of events, as well as virus and host components, that regulate ER membrane penetration. Mutant virus and inhibitor studies demonstrate VP3 and likely the viral genome, as well as cellular proteasome, control ER-to-cytosol transport. This large particle subsequently disassembles in the cytosol. However, despite these ER-dependent conformational changes, SV40 crosses the ER membrane as a large and intact particle consisting of the VP1 coat, the internal components VP2, VP3, and the genome. We found that, upon ER arrival, SV40 is released into the lumen and undergoes sequential disulfide bond disruptions to reach the cytosol. A cell-based assay was used to probe the endoplasmic reticulum (ER)-to-cytosol membrane transport of the non-enveloped SV40. Non-enveloped viruses penetrate host membranes to infect cells.
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