The role of septins during vaccinia virus spread

Septins are highly conserved components of the cytoskeleton found in animals and fungi. They play a variety of roles in key cellular processes including cell division, cell migration and membrane trafficking. During host-pathogen interactions, septins inhibit bacterial infection by forming cage-like structures around pathogens such as Shigella. In addition, two recent genome-wide RNAi screens demonstrated that septins play an undefined role during vaccinia virus replication. Utilizing cell-based assays and microscopy I set out to determine the role of septins in vaccinia infected cells. I found that septins are recruited to vaccinia virus immediately following its fusion with the plasma membrane during viral egress. Live cell imaging reveals that septins are lost from beneath the virus once the virus stimulates Arp2/3 complex-dependent actin polymerization to enhance its cell-to-cell spread. Virus-induced actin polymerization involves the phosphorylation of the viral protein A36, leading to the recruitment of Cdc42, Nck, Grb2, WIP and N-WASP, which activate the Arp2/3 complex. Chemical or genetic inhibition of A36 phosphorylation dramatically increases the number of virus particles co-localizing with septins. Further experiments demonstrate that the recruitment of Nck and subsequently dynamin, but not Grb2, WIP:N-WASP or the Arp2/3-complex, promote the loss of septins from virions. RNAi-mediated depletion of septins increases virus release, accelerates cell-to-cell spread, and induces more robust actin tails. Collectively, my results demonstrate that septins limit the spread of vaccinia infection in cell monolayers and the recruitment of dynamin downstream of Nck enables the virus to overcome septin-mediated restriction. This is the first example of septins having an anti-viral effect and my work identifies a new role for septins in host defence