Proteolytic Cleavage of VP1-2 Is Required for Release of Herpes Simplex Virus 1 DNA into the Nucleus

Abstract

In this report we propose a model in which after the herpes simplex virus (HSV) capsid docks at the nuclear pore, the tegument protein attached to the capsid must be cleaved by a serine or a cysteine protease in order for the DNA to be released into the nucleus. The proteolytic cleavage of VP1-2 occurs only after the capsid is attached to the nuclear pore. Thus, TPCK prevented the release of HSV-1 DNA into the nucleus when added to medium 1 hour after infection with tsB7 at 39.5°C followed by a shift down to the permissive temperature. The ts lesion maps in the U L 36 gene. At the nonpermissive temperature, the capsids accumulate at the nuclear pore but the DNA is not released into the nucleus.In this report we present supporting evidence for a model for the release of herpes simplex virus 1 (HSV-1) DNA from the capsid into the nucleus of an infected cell. Briefly, following the attachment of the capsid to the nuclear pore, VP1-2, the largest tegument protein, undergoes a proteolytic cleavage that cleaves a 55-kDa N-terminal fragment. This proteolytic cleavage is essential for the subsequent conformational changes in the capsid that allow the release of the HSV-1 DNA into the nucleus. Relevant to the presentation of the data supporting the model are the following.HSV-1 initiates infection by attachment to the cell membrane via the interaction between viral glycoproteins gB and gC with heparan sulfate (39, 43). The initial attachment is followed by the fusion of the viral envelope with the plasma membrane, a process initiated by the interaction of gD with a specific receptor and mediated by gB, gH, and gL (11,24,35). The capsid with associated tegument proteins is released into the cytosol and transported via dynein and microtubule-directed transport toward the nucleus (40). During the transport some tegument proteins, most notably VP16 (2) and virion host shutoff protein (23), the products of U L 48 and U L 41, respectively, are released, whereas some, such as VP1-2, the product of the U L 36 gene, remain associated with the capsid (16,44). Once the capsid arrives at the nuclear pore, viral DNA is rapidly released into the nucleus to enable the expression of its genes. Information regarding the process of attachment to the nuclear pore and the release of viral DNA into the nucleus is scarce. Attachment of capsid to the nuclear pores and translocation of viral DNA into the nucleus require im-

Herpes Simplex Virus Replication: Roles of Viral Proteins and Nucleoporins in Capsid-Nucleus Attachment

“…Our studies support the hypothesis that capsids attach to the NPC by way of an interaction with Nup358. Once bound to the NPC, a protease of unknown origin cleaves the tightly associated tegument, inducing a change that releases the viral DNA (26). HSV-1 genome uncoating resembles uncoating exhibited by double-stranded DNA bacteriophage, with the genome being extruded through the opening of a channel at a unique portal-containing vertex (35).…”

“…The mutation maps to UL36, the gene encoding VP1/2 (4). Following nuclear capsid binding, VP1/2 is cleaved to allow uncoating to proceed (26). VP1/2's role in nuclear binding and the possible involvement of other herpesvirus proteins are not well understood.…”

“…The cellular factors importin-␤ and Ran-GTP are essential for binding (38), but neither the nucleoporins nor the viral proteins involved have been identified. VP1/2 has recently been shown to play a role in uncoating, long implied by the tsB7 mutant (26). tsB7 HSV-1 has a defect that allows nuclear capsid binding but not uncoating at the nonpermissive temperature (4,45).…”

Dynamic ubiquitination drives herpesvirus neuroinvasion

“…pUL36 is attached directly to the icosahedral capsid shell of these enveloped viruses (10)(11)(12) and, due to its location between the capsid and envelope, is referred to as a herpesvirus tegument protein. Upon virus entry into a cell, pUL36 becomes exposed to the cytosol (4,13,14) and directs delivery of the capsid and its DNA content to the nucleus (2,(15)(16)(17)(18)(19). Following replication, pUL36 is also essential for viral assembly and egress (20)(21)(22).…”

Virus entry at a glance

“…They have evolved mechanisms to release only their DNA genome through NPCs (Greber et al, 1997;Jovasevic et al, 2008;Ojala et al, 2000;Pasdeloup et al, 2009;Preston et al, 2008;Strunze et al, 2011). Viruses with smaller capsids, such as hepatitis B and polyoma viruses, can enter through the pores in either an intact or modified form (Kann et al, 1999;Nakanishi et al, 1996;Qu et al, 2004;Rabe et al, 2009;Rabe et al, 2003).…”