Herpes simplex virus nucleocapsids mature to progeny virions by an envelopment --> deenvelopment --> reenvelopment pathway

Abstract

Herpes simplex virus (HSV) nucleocapsids acquire an envelope by budding through the inner nuclear membrane, but it is uncertain whether this envelope is retained during virus maturation and egress or whether mature progeny virions are derived by deenvelopment at the outer nuclear membrane followed by reenvelopment in a cytoplasmic compartment. To resolve this issue, we used immunogold electron microscopy to examine the distribution of glycoprotein D (gD) in cells infected with HSV-1 encoding a wild-type gD or a gD which is retrieved to the endoplasmic reticulum (ER). In cells infected with wild-type HSV-1, extracellular virions and virions in the perinuclear space bound approximately equal amounts of gD antibody. In cells infected with HSV-1 encoding an ER-retrieved gD, the inner and outer nuclear membranes were heavily gold labeled, as were perinuclear enveloped virions. Extracellular virions exhibited very little gold decoration (10- to 30-fold less than perinuclear virions). We conclude that the envelope of perinuclear virions must be lost during maturation and egress and that mature progeny virions must acquire an envelope from a post-ER cytoplasmic compartment. We noted also that gD appears to be excluded from the plasma membrane in cells infected with wild-type virus.

FIG. 1

Immunogold labeling of cells infected with HSV-1 expressing wild-type gD. (A) Decoration of inner and outer nuclear membranes and particles in the perinuclear space; (B and C) decoration of extracellular particles. Bars, 100 nm.

FIG. 2

Immunogold labeling of cells infected with HSV-1 expressing an ER-targeted gD. (A) Decoration of particles in the perinuclear space; (B and E) decoration of inner and outer nuclear membranes; (D) enlargement of the particle shown in micrograph A that appears to be budding through the inner nuclear membrane; (C and F) extracellular particles. The arrows in micrographs B, D, and E indicate unenveloped capsids in the nucleus. The arrow in micrograph F indicates a very rare example of a decorated extracellular particle. Bars, 100 nm.

FIG. 2

Immunogold labeling of cells infected with HSV-1 expressing an ER-targeted gD. (A) Decoration of particles in the perinuclear space; (B and E) decoration of inner and outer nuclear membranes; (D) enlargement of the particle shown in micrograph A that appears to be budding through the inner nuclear membrane; (C and F) extracellular particles. The arrows in micrographs B, D, and E indicate unenveloped capsids in the nucleus. The arrow in micrograph F indicates a very rare example of a decorated extracellular particle. Bars, 100 nm.