Like hobos on a train, HIV, the virus that causes AIDS, uses a pre-existing transport system to leave one infected cell and infect new ones, Hopkins scientists have discovered.
Their findings, published in the June issue of Plos Biology, counter the prevailing belief that HIV and other retroviruses can only leave and enter cells by virus-specific mechanisms.
“It appears that cells make HIV and other retroviruses by a naturally occurring export mechanism,” says Stephen Gould, Ph.D., Professor of Biological Chemistry at Johns Hopkins. Cells normally export certain membrane-bound molecules to the outside world by means of small sacs known as exosomes. By studying human T-cells under a microscope, Gould, Yi Fang, Ning Wu, and other members of his team discovered what's needed to qualify proteins for exosomal travel.
“Surprisingly, all that's needed for a protein to get out of the cell in exosomes are the ability to clump together and attach to the cell's membrane,” Gould says.
In one experiment, Gould and his team added chemicals to normal human cells that force nearby proteins together into a clump, and this was enough to get them sent out of the cell in exosomes. If they added a tether to force naturally-clumping proteins inside the cell to the membrane, the proteins met a similar exosomal deportation fate.
The major HIV protein ‘Gag' has both of these properties that cells sense in selecting exosomal cargoes. When the researchers removed the tethers or clumping signals from Gag it could no longer get out of the cell. However, if they were replaced with synthetic membrane anchors and clumping domains Gag regained its ability to get out of cells in exosomes.