November 17, 2011

A virus is both crafty and determined…pretty good for a non-living thing, right? Today’s image is from a paper discussing transport of the simian virus 40, which has some clever tricks up its little capsid sleeve to make the host cell help out with the viral infection.

Simian virus 40 (SV40) is a DNA virus that does not have its own membrane envelope, making transport into and around a cell a non-trivial task. SV40 first binds to a cell’s membrane, then uses endocytosis to enter the cell and travel to the endoplasmic reticulum (ER). From the ER, the virus penetrates the membrane to have access to the cytosol of the cell, where it can then reach the nucleus and replicate its genome. A recent paper describes this ER membrane penetration step, and reports that SV40 particles go through a major structural change within the lumen of the ER. According to Geiger and colleagues, this conformational change exposes a part of the minor viral protein VP2, which then embeds the virus into the ER membrane. This change attracts the cell’s own ERAD machinery, normally used to find and degrade misfolded proteins in the ER, which then allows the escape of the virus particles into the cytosol. Images above are electron micrographs of cells infected with SV40 for 2 (left), 6 (right), and 19 (bottom) hours. Early in the infection, virus particles are bound to endosomal or vesicle membranes (left, arrows). In the ER, the virus appears more compact (right, red arrows), indicating the structural change. Later, the ER membrane appears flattened around the virus particles (bottom, arrows).

Geiger, R., Andritschke, D., Friebe, S., Herzog, F., Luisoni, S., Heger, T., & Helenius, A. (2011). BAP31 and BiP are essential for dislocation of SV40 from the endoplasmic reticulum to the cytosol Nature Cell Biology, 13 (11), 1305-1314 DOI: 10.1038/ncb2339
Adapted by permission from Macmillan Publishers Ltd, copyright ©2011