Herpesviruses deform membrane during capsid nuclear export by lipid ordering and protein scaffolding.

Abstract

Herpesviruses are large viruses that infect nearly all vertebrates and some invertebrates and cause lifelong infections in most of the world's population. A critical step in their replication is the export of the viral capsids from the nucleus into the cytoplasm, which occurs by an unusual mechanism termed nuclear egress. Too large to fit through nuclear pores, capsids instead bud at the inner nuclear membrane to form perinuclear enveloped virions, which then fuse with the outer nuclear membrane, releasing the capsids into the cytoplasm. This process is mediated by the virus-encoded nuclear egress complex (NEC) that deforms the membrane around the capsid. To understand how the NEC generates negative membrane curvature, we reconstituted the membrane budding process in vitro using the NEC from herpes simplex virus 1, a prototypical herpesvirus that causes cold sores. To probe its mechanism, we employed a combination of confocal microscopy, electron spin resonance, and structural biology. We found that the NEC uses clusters of positively charged residues to insert into the lipid headgroups, which increases lipid order. We also found that the NEC oligomerizes into a membrane-bound hexagonal coat on the inner surface of the budded vesicles. We propose that the NEC combines lipid ordering and oligomerization to mold the membrane into a spherical shape. Whereas lipid ordering generates negative membrane curvature locally, the NEC oligomerization into a hexagonal scaffold is necessary to achieve negative membrane curvature over a large membrane area. Similar principles may underlie membrane deformation in other systems. Our findings provide a biophysical explanation for the phenomenon of virus-induced nuclear budding.