Herpes simplex virus membrane proteins gE/gI and US9 act cooperatively to promote transport of capsids and glycoproteins from neuron cell bodies into initial axon segments

Herpes simplex virus (HSV) and other alphaherpesviruses must move from sites of latency in ganglia to peripheral epithelial cells. How HSV navigates in neuronal axons is not well understood. Two HSV membrane proteins, gE/gI and US9, are key to understanding the processes by which viral glycoproteins, unenveloped capsids, and enveloped virions are transported toward axon tips. Whether gE/gI and US9 function to promote the loading of viral proteins onto microtubule motors in neuron cell bodies or to tether viral proteins onto microtubule motors within axons is not clear. One impediment to understanding how HSV gE/gI and US9 function in axonal transport relates to observations that gE^-, gI^-, or US9^- mutants are not absolutely blocked in axonal transport. Mutants are significantly reduced in numbers of capsids and glycoproteins in distal axons, but there are less extensive effects in proximal axons. We constructed HSV recombinants lacking both gE and US9 that transported no detectable capsids and glycoproteins to distal axons and failed to spread from axon tips to adjacent cells. Live-cell imaging of a gE^-/US9^- double mutant that expressed fluorescent capsids and gB demonstrated > 90% diminished capsids and gB in medial axons and no evidence for decreased rates of transport, stalling, or increased retrograde transport. Instead, capsids, gB, and enveloped virions failed to enter proximal axons. We concluded that gE/gI and US9 function in neuron cell bodies, in a cooperative fashion, to promote the loading of HSV capsids and vesicles containing glycoproteins and enveloped virions onto microtubule motors or their transport into proximal axons.