Host Vesicle Fusion Proteins VAPB, Rab11b and Rab18 Contribute to HSV-1 Infectivity by Facilitating Egress through the Nuclear Membrane

The herpesvirus process of primary envelopment and de-envelopment as viral particles exit the nucleus has been for many years one of the least understood steps in the virus life cycle. Though viral proteins such as pUL31, pUL34, pUS3 and others are clearly important, these are likely insufficient for efficient fusion with the nuclear membrane. We postulated that host nuclear membrane proteins involved in virus nuclear egress would move from the inner to outer nuclear membranes due to membrane fusion events in primary envelopment and de-envelopment and then diffuse into the endoplasmic reticulum. Membrane fractions were prepared enriched in the nuclear envelope or the endoplasmic reticulum with and without HSV-1 infection and analyzed by mass spectrometry, revealing several vesicle fusion proteins as candidates in the viral nuclear egress pathway. Knockdown of three of these, VAPB, Rab11b, and Rab18, significantly reduced titers of released virus while yielding nuclear accumulation of encapsidated particles. Antibody staining revealed that VAPB visually accumulates in the inner nuclear membrane during HSV-1 infection. VAPB also co-localizes at early time points with the viral pUL34 protein known to be involved in nuclear egress. Most strikingly, VAPB was also observed on HSV-1 virus particles by immunogold labelling electron microscopy. Thus, these data reveal several new host cell vesicle fusion proteins involved in viral nuclear egress.