In their evolutionary battle for survival, viruses have developed strategies to spark and perpetuate infection. Once inside a host cell, the Ebola virus, for example, hijacks molecular pathways to replicate itself and eventually make its way back out of the cell into the bloodstream, where it can spread further.
But our own cells, in the case of Ebola and many other viruses, aren't without defenses. In a study published in the Proceedings of the National Academy of Sciences, a team led by University of Pennsylvania School of Veterinary Medicine scientists discovered a way human cells hamper the Ebola virus' ability to exit.
An interaction between viral and host proteins prompts host cells to ramp up activity of a pathway responsible for digesting and recycling proteins, the team found. This activity, known as autophagy "self-eating," allows fewer viral particles to reach the surface of a host cell, thus reducing the number that can exit into the bloodstream and further propagate infection.
"This interaction seems to be part of an innate defense mechanism," says Ronald N. Harty, a professor at Penn Vet and senior author on the study. "Human cells appear to specifically target a key Ebola virus protein and direct it into the autophagy pathway, which is how cells process and recycle waste."
The investigation emerged from a longtime area of focus for Harty's lab: the interaction between the viral protein VP40, found in both Ebola and Marburg viruses, and various human proteins. In the group's previous work, they've found that one area of VP40, known as a PPXY motif, binds corresponding motifs known as WW domains on specific host proteins.
In many instances, this PPXY-WW interaction causes more viral particles to exit the cell in a process called "budding." But in screening various host proteins thought to play a role in the process, Harty and postdoc Jingjing Liang, the study's lead author, uncovered some that did the opposite upon binding VP40, causing budding to decrease. One of these was a protein called Bag3, on which they reported in a PLOS Pathogens paper in 2017.
Though Ebola is a potentially deadly virus, Harty and colleagues can safely study its workings in a Biosafety Level 2 laboratory, substituting virus-like particles (VLPs) that express VP40 for the virus itself. These VP40 VLPs are not infectious but can bud out from host cells like the real thing.
In the new work, the Penn Vet researchers and colleagues from the Texas Biomedical Research Institute dug deeper to learn about the mechanism by which Bag3 reduced budding. Bag3 is known as a "co-chaperone" protein, involved in forming a complex with other proteins and chaperoning them on their trip to be digested, ultimately in organelles called autolysosomes, part of the process of autophagy. Using VP40 VLPs, Harty's group confirmed that VP40 bound to Bag3 and formed the protein complex. When the researchers added a compound that is known to block formation of this complex, they saw VP40 being released; VLP budding activity subsequently increased.
