Tegument Assembly and Secondary Envelopment of Alphaherpesviruses

Abstract

Alphaherpesviruses like herpes simplex virus are large DNA viruses characterized by their ability to establish lifelong latent infection in neurons. As for all herpesviruses, alphaherpesvirus virions contain a protein-rich layer called "tegument" that links the DNA-containing capsid to the glycoprotein-studded membrane envelope. Tegument proteins mediate a diverse range of functions during the virus lifecycle, including modulation of the host-cell environment immediately after entry, transport of virus capsids to the nucleus during infection, and wrapping of cytoplasmic capsids with membranes (secondary envelopment) during virion assembly. Eleven tegument proteins that are conserved across alphaherpesviruses have been implicated in the formation of the tegument layer or in secondary envelopment. Tegument is assembled via a dense network of interactions between tegument proteins, with the redundancy of these interactions making it challenging to determine the precise function of any specific tegument protein. However, recent studies have made great headway in defining the interactions between tegument proteins, conserved across alphaherpesviruses, which facilitate tegument assembly and secondary envelopment. We summarize these recent advances and review what remains to be learned about the molecular interactions required to assemble mature alphaherpesvirus virions following the release of capsids from infected cell nuclei.

Keywords: HSV-1; PrV; herpes simplex virus; pseudorabies virus; virus egress; virus maturation.

Figure 1

Neuronal trafficking during entry and egress. Alphaherpesviruses establish latent infection in the nuclei of peripheral ganglia following retrograde transport of capsids along microtubules. Reactivation results in the production of new virions that undergo anterograde trafficking back to peripheral tissues. The assembly state of viral particles prior to anterograde axonal transport is disputed and two models have been proposed: the “married model” predicts that virions are assembled in the cell body and trafficked within vesicles; the “separate model” predicts that capsids and secondary-envelopment membranes are trafficked separately with final virion assembly occurring at or near the sites of egress. Minus-end directed transport to the cell body along microtubules is driven by dynein while kinesins drive plus-end directed transport to the cell periphery. The movement of viral particles along axons during entry and egress is bidirectional and saltatory suggesting that both dynein and kinesin motor proteins may be involved. How the net direction of transport during entry and egress is determined is currently unknown.

Figure 2

Maturation and egress of herpesviruses. Replication of the viral genome and encapsidation occurs in the nucleus. Once assembled, capsids interact with the inner nuclear membrane and bud into the perinuclear space where they form primary enveloped particles. The primary envelope is then lost upon fusion with the outer nuclear membrane and unenveloped capsids are released into the cytoplasm. Cytoplasmic capsids acquire tegument proteins and their membrane by budding into specialised vesicles, probably derived from endosomes or the trans-Golgi network (TGN), that are studded with viral glycoproteins and outer tegument proteins. The secondary envelopment step also provides a transport vesicle that later fuses with the plasma membrane (PM) to release enveloped virions from the cell.

Figure 3

Conserved alphaherpesvirus tegument proteins (blue) link the capsid (yellow) to the glycoproteins and envelope proteins (green) embedded in the virion lipid bilayer envelope (grey). Tegument assembles via a dense network of protein:protein interactions: solid lines indicate interactions demonstrated in HSV and dashed lines show interactions demonstrated for PrV. Some tegument proteins associate directly with the envelope via post-translational modifications conferring lipophilic palmitoyl (red) or myristoyl (purple) groups. The proteins that comprise the portal vertex associated tegument (PVAT) are currently undefined.

Figure 4

Protein pUL36 extends from capsid vertices and interacts with the capsid vertex-specific component (CVSC). (Top inset) The extended N-terminal region of pUL36 interacts with pUL37 and pUL48. For clarity pUL36 and pUL37 are not drawn to scale. (Bottom inset) Recent studies of HSV, PrV and KSHV [107,135,145] suggest that CVSC component pUL25 lies over the penton vertex, pUL17 lies above the penton proximal pUL18-pUL38 triplex, a C-terminal region of pUL36 contributes to the CVSC density, and that pUL36 is essential for CVSC formation.

Figure 5

Proteins pUL11, pUL16 and pUL21 may form a tripartite complex that binds gE. The C-terminal domain of pUL16 inhibits its ability to co-localise with pUL11 and gE, co-localization of pUL16 with pUL11 is enhanced in the presence of pUL21, and the presence of pUL11 promotes co-localization of pUL16 and gE [101]. An alternative model is that pUL16 acts as a molecular chaperone, promoting the correct folding of pUL11, pUL21 and/or gE.