Selection of HSV capsids for envelopment involves interaction between capsid surface components pU _L 31, pU _L 17, and pU _L 25

Abstract

During egress from the nucleus, HSV capsids that contain DNA (termed C capsids) are preferentially enveloped at the inner nuclear membrane over capsid types lacking DNA. Using coimmunoprecipitation and biochemical analyses of wild-type and mutant capsids, we identify an interaction between a complex of pU L 17/ pU L 25, termed the C capsid-specific complex (CCSC), and pU L 31, a component of the nuclear egress complex (NEC). We also show that the interactions between these components are dependent on expression of all three proteins but occur independently of the pU L 31 interacting protein and NEC component pU L 34, as well as a kinase encoded by U S 3 that phosphorylates both pU L 31 and pU L 34. The interaction between the CCSC and pU L 31 in the NEC suggests a mechanism to conserve viral resources by promoting assembly of only those viral particles with the potential to become infectious. virus assembly | virus egress H erpesvirus nucleocapsids (type C capsids) are assembled in the nucleoplasm and are preferentially selected over other capsid types, such as type B that lack DNA, to undergo an initial or primary envelopment reaction at the inner nuclear membrane (INM) (reviewed in refs. 1 and 2). Primary envelopment requires the products of genes U L 31 and U L 34 in the HSV system (3-5). Orthologs of U L 31 and U L 34 are present in all known herpesviruses, and for those systems in which it has been studied, the requirement for these proteins in primary envelopment is conserved (6-9). U L 31 encodes a nucleoplasmic phosphoprotein, pU L 31 (10), that is maintained in close approximation to the INM by association with pU L 34, a type II integral membrane protein (5,(11)(12)(13). The bulk of pU L 34 is located within the nucleoplasm, with only five amino acids predicted to lie within the perinuclear space (14, 15). Although it has been established that pU L 31 and pU L 34 are incorporated into perinuclear virions (16,17), whether the capsid engages the pU L 31/pU L 34 complex (also known as the nuclear envelopment complex or NEC) directly or indirectly is not known.The U L 17 and U L 25 gene products interact, forming a stable complex (18). DNA-containing C capsids contain ≈75 copies of pU L 25, whereas B capsids contain ≈20 copies (10). Because of its enrichment in C capsids, the U L 25/U L 17 complex was named the C capsid-specific complex or CCSC (19). The CCSC bridges pentameric vertices to the adjacent 20 planar faces on the capsid surface (10,19,20). One hypothesis to explain how C capsids are selected for envelopment is that the products of U L 25 and U L 17 bind more efficiently to the surface of type C capsids after DNA packaging is complete (19), and these capsids subsequently engage the NEC complex either directly or indirectly. Consistent with this hypothesis is the observation that U L 17 and U L 25 null capsids do not become enveloped (21,22). However, CCSC components have additional functions that may contribute indirectly to envelopment. For example, U L 17 is necessary for DNA to be cleaved and...

Herpes Simplex Virus 1 UL47 Interacts with Viral Nuclear Egress Factors UL31, UL34, and Us3 and Regulates Viral Nuclear Egress

“…Taken together, these results indicated that UL47 formed a complex with UL34, UL31, and/or Us3 in HSV-1-infected cells. This conclusion was in agreement with previous reports (12,21,24) that, in HSV-1-infected cells, UL47 interacted with viral capsid protein UL17, which further formed a complex with UL31 and viral capsid protein UL25, and that UL47 interacted with Us3. At present it remains to be determined whether UL47, UL31, UL34, and Us3 form a high-order complex in HSV-…”

“…This hypothesis was supported by the observation in this study that UL47 interacted with UL31 and UL34 in HSV-1-infected cells and that UL47 was a component of primary enveloped virions, which enabled UL47 to interact with the UL34/UL31 complex during primary envelopment. As described above, UL47 was reported to form a complex with UL17, a component of the HSV-1 CVSC, and, therefore, may interact with UL31 to recruit HSV nucleocapsids for primary envelopment (12,13). Therefore, UL47 may upregulate the primary envelopment of nucleocapsids by promoting recruitment of nucleocapsids through interaction with the UL17/UL25/UL31 complex, which may stabilize the association of capsids with the UL34/UL31 complex at the nuclear membrane.…”

“…(ii) UL47 was also shown to interact with capsid protein UL17 (24). As described above, UL17 forms a CVSC complex with UL25, which was suggested to recruit nucleocapsids for primary envelopes by interacting with the UL31/UL34 complex (12,13). (iii) Recently, it has been reported that UL47 interacted with the viral endoribonuclease responsible for virus host protein synthesis shutoff (vhs) and attenuated vhs activity (25).…”

“…In the absence of the HSV-1 UL31/UL34 complex, nucleocapsids accumulate in the nucleoplasm, and progeny virus intermediates and virions are barely detectable in the perinuclear space or cytoplasm or at the cell surface (5,6). The HSV-1 UL31/UL34 complex and its homologs in other herpesviruses have been suggested to coordinate multiple events in the primary envelopment of nucleocapsids, including the following: (i) disruption of the nuclear lamina, which has been suggested to facilitate herpesvirus nucleocapsid access to the INM, by recruiting cellular protein kinases, such as protein kinase C isoforms, and by direct binding to components of the nuclear lamina (i.e., lamins A and C) and modifying their conformation (1, 2, 7-11); (ii) recruitment of nucleocapsids into primary envelopes by interaction of the UL31/UL34 complex and the capsid vertex-specific component (CVSC), which consists of the conserved capsid proteins UL17 and UL25 (12,13); and (iii) budding of nucleocapsids into the INM (14,15). In addition to UL31 and UL34, an HSV-1 serine/threonine protein kinase Us3 has been suggested to be involved in HSV-1 primary envelopment since Us3 phosphorylates and regulates proper localization of UL34 and UL31 at the nuclear membrane (4,16,17) and phosphorylates and modifies lamins A and C (7,8,10).…”

Viral and cellular subnuclear structures in human cytomegalovirus-infected cells

“…It is unknown what mechanism selects only DNA-containing capsids for nuclear egress in HCMV-infected cells. In HSV-infected cells, this process is governed by the interaction of the nuclear egress complex and capsid proteins (Yang & Baines, 2011). A similar mechanism may operate in HCMV-infected cells.…”