Nuclear Exodus: Herpesviruses Lead the Way

Abstract

Most DNA viruses replicate in the nucleus and exit it either by passing through the nuclear pores or by rupturing the nuclear envelope. Unusually, herpesviruses have evolved a complex mechanism of nuclear escape whereby nascent capsids bud at the inner nuclear membrane to form perinuclear virions that subsequently fuse with the outer nuclear membrane, releasing capsids into the cytosol. Although this general scheme is accepted in the field, the players and their roles are still debated. Recent studies illuminated critical mechanistic features of this enigmatic process and uncovered surprising parallels with a novel cellular nuclear export process. This review summarizes our current understanding of nuclear egress in herpesviruses, examines the experimental evidence and models, and outlines outstanding questions with the goal of stimulating new research in this area.

Keywords: budding; de-envelopment; envelopment; herpesvirus; nuclear egress; scission; structure; viral egress.

Figure 1

Overview of herpesvirus egress. Viral nucleocapsids are assembled in the nucleus. To relocate into the cytoplasm, they bud at the inner nuclear membrane. This process is mediated by the nuclear egress complex. Resulting primary virions are translocated to the perinuclear space, where they fuse with the outer nuclear membrane. Naked nucleocapsids are released into the cytoplasm to undergo final maturation and acquire a tegument protein layer. Cytoplasmic capsids bud at membranes derived from the trans Golgi network or early endosomes and thereby obtain an envelope containing the glycoproteins needed for cell entry. Mature infectious virions hijack the secretory pathway to be released into the extracellular space.

Figure 2

Structural features of the nuclear egress complex of (a) herpes simplex virus 1 (HSV-1; PDB: 4ZXS) and (b) pseudorabies virus (PRV; PDB: 4Z3U). UL31 (orange) and UL34 (teal) form an elongated complex, with UL31 wrapping its N-terminal hook around UL34. The two molecules interact extensively, which implies high binding affinity. The membrane-proximal end is located at the bottom of the heterodimer in this orientation. The regions important for membrane interaction are missing from the structure and are indicated schematically, along with the membrane. The highly conserved surface patch on the membrane-distal end of UL31 is the putative capsid-binding site.

Figure 3

Multiple proteins are involved in primary envelopment (nuclear budding). Although the nuclear egress complex can mediate vesicle budding by itself in vitro, this process appears subject to positive and negative regulation by a number of proteins during infection. Cellular and viral kinases (e.g., UL13, US3, and PKC) are recruited to the sites of primary envelopment for herpesviruses of all subfamilies. They phosphorylate proteins within the nuclear lamina as well as in the nuclear egress complex itself. This loosens the stiff lamina and allows capsids to be recruited to the inner nuclear membrane. Other viral proteins, such as UL47, have been implicated in regulating efficient nuclear egress, but their precise roles are still unclear. The nuclear egress complex is also responsible for capsid recruitment and may do so by binding the accessory capsid proteins UL17/UL25 or the major capsid protein VP5.

Figure 4

Nuclear egress complex coat formation drives membrane deformation and budding. (a) UL34 (teal), anchored in the inner nuclear membrane, binds UL31 (orange) to form the nuclear egress complex heterodimer. The complex initially forms hexameric rings that eventually extend into a larger lattice when the capsid approaches. Conformational changes likely deform the membrane initially, but larger structural rearrangements are needed for sphere formation. This is possibly achieved by the introduction of errors within the hexagonal lattice. (b) The herpes simplex virus (HSV)-1 nuclear egress complex forms crystalline hexagonal lattices (PDB: 4ZXS), which resemble the hexagonal coats observed by cryo–electron tomography. The detailed analysis of the lattice allowed the identification of regions important for coat formation and membrane budding. Figure adapted with permission from Reference .

Figure 5

De-envelopment at the outer nuclear membrane. It is still largely unclear how de-envelopment of herpesvirus capsids is mediated. Viral glycoproteins that are required for viral entry do not play the same role here, suggesting that an unidentified cellular fusion machinery drives the de-envelopment process. Some viral proteins, such as US3 and UL51, may be involved in the process, but their roles are unclear. US3 kinase activity may aid nuclear egress complex lattice disassembly prior or coincident with membrane fusion.