Differing Roles of Inner Tegument Proteins pUL36 and pUL37 during Entry of Herpes Simplex Virus Type 1

Abstract

Studies with herpes simplex virus type 1 (HSV-1) have shown that secondary envelopment and virus release are blocked in mutants deleted for the tegument protein gene UL36 or UL37, leading to the accumulation of DNA-containing capsids in the cytoplasm of infected cells. The failure to assemble infectious virions has meant that the roles of these genes in the initial stages of infection could not be investigated. To circumvent this, cells infected at a low multiplicity were fused to form syncytia, thereby allowing capsids released from infected nuclei access to uninfected nuclei without having to cross a plasma membrane. Visualization of virus DNA replication showed that a UL37-minus mutant was capable of transmitting infection to all the nuclei within a syncytium as efficiently as the wild-type HSV-1 strain 17؉ did, whereas infection by UL36-minus mutants failed to spread. Thus, these inner tegument proteins have differing functions, with pUL36 being essential during both the assembly and uptake stages of infection, while pUL37 is needed for the formation of virions but is not required during the initial stages of infection. Analysis of noninfectious enveloped particles (L-particles) further showed that pUL36 and pUL37 are dependent on each other for incorporation into tegument.

Nuclear Egress of Pseudorabies Virus Capsids Is Enhanced by a Subspecies of the Large Tegument Protein That Is Lost upon Cytoplasmic Maturation

“…While these studies demonstrate that the VP1/2 C-terminal isoform performs a supportive role during nuclear egress, the isoform lacks regions that are required for continued virion morphogenesis in the cytoplasm (1, 2, 39, 48). Because VP1/2 is essential for the cytoplasmic envelopment of capsids that have egressed from the nucleus, the C-terminal capsid-bound fragment presumably must be replaced by the full-length protein prior to cytoplasmic envelopment and exocytosis from the cell (20,22,65). The absence of the nuclear VP1/2 isoforms in virions is documented in Fig.…”

“…Recombinant HSV-1 or PRV lacking VP1/2 fails to undergo cytoplasmic envelopment, resulting in the accumulation of capsids in the cytosol (20,22). These findings document the importance of VP1/2 in cytoplasmic envelopment, but they have also led to the interpretation that VP1/2-null viruses are not impaired in nuclear egress (20,65,67). While it is uncontested that VP1/2 is nonessential for nuclear egress, measuring nuclear egress efficiency in the absence of VP1/2 is complicated by the pleiotropic effect of the deletion: the accumulation of cytosolic capsids that cannot proceed to final envelopment could obscure a reduced rate of egress from the nucleus.…”

“…Failed capsids accumulate in at least two forms that are readily isolated from infected cell nuclei: those that never expel the protein scaffold (also known as B capsids) and those that expel the scaffold but do not retain full-length genomes (also known as A capsids) (8,25,57). Nevertheless, packaged nucleocapsids (also known as C capsids) predominate in extracellular viral particles, and this selectivity is imparted by an inability of A and B capsids to efficiently egress from the nucleus (13,25,52,62,65,76,89). The mechanism of nuclear egress selectivity is a topic of much discussion given that the obvious difference between the three capsid species is hidden inside the capsid shell.…”

Role of tegument proteins in herpesvirus assembly and egress

“…It forms a complex with pUL37 as part of the capsid-associated inner tegument. Deletion of either of the genes encoding pUL36 or pUL37 blocks further tegumentation of the capsid in the cytoplasm (Desai, 2000;Desai et al, 2001;Klupp et al, 2001b;Fuchs et al, 2004;Leege et al, 2009;Roberts et al, 2009). By interacting with pUL48, pUL36 is a major factor for recruiting pUL48 onto capsids (Ko et al, 2010).…”

DNA-tumor virus entry—From plasma membrane to the nucleus

“…UL36 (VP1/2) together with UL37 and UL25 is one of the innermost capsid associated tegument proteins that remain associated with the incoming capsids until docking at the NPC. UL36 deletion mutants failed to spread the infection to uninfected nuclei in experimentally induced syncitia indicating that UL36 was required for capsid transport to, or DNA import into the nucleus (162). Interestingly, proteolytic cleavage of VP1/2 was required for DNA import into the nucleus but not for capsid docking to the NPC (163).…”