Viruses typically hitchhike around the cytoplasm on microtubules. Ohkawa et al. reveal that one type of virus can travel within the cell using actin filaments.
Once baculoviruses break into a cell, they need a lift to the nucleus so their genetic material can be copied. However, these viruses don't require microtubules for the journey. Ohkawa et al. investigated whether the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) relies on actin instead.
The researchers observed tagged viruses scooting through the cell at the tips of elongating actin filaments. A viral protein called P78/83 triggers actin polymerization by activating the Arp2/3 complex. Unable to steer, a virus keeps going until it runs into the nucleus. Actin polymerization continues even after the collision, driving the virus into the nuclear membrane hard enough to make a dent. However, the researchers don't think that the virus forces its way through the membrane. When they blocked transport through nuclear pores, the number of viral particles in the nucleus plunged 78%, suggesting that the virus enters via these passageways.
The vaccinia virus uses actin to cruise across the cell surface, but AcMNPV is the first virus shown to travel aboard actin within the cell. Actin also helps the virus make a move that might sustain an infection. Clusters of AcMNPV particles invade a cell and then break up after entry. Some of the viral particles head for the nucleus. Once the viral takeover of the cell is well under way, some viruses return to the plasma membrane and exit. The team showed that actin transports these viruses to the membrane. Even if the host spurs the cell to undergo apoptosis, these escapees survive to infect again.