Human Cytomegalovirus pUL93 Links Nucleocapsid Maturation and Nuclear Egress

Abstract

Human cytomegalovirus (HCMV) pUL93 and pUL77 are both essential for virus growth, but their functions in the virus life cycle remain mostly unresolved. Homologs of pUL93 and pUL77 in herpes simplex virus 1 (HSV-1) and pseudorabies virus (PRV) are known to interact to form a complex at capsid vertices known as the capsid vertex-specific component (CVSC), which likely stabilizes nucleocapsids during virus maturation and also aids in nuclear egress. In herpesviruses, nucleocapsids assemble and partially mature in nuclear replication compartments and then travel to the inner nuclear membrane (INM) for nuclear egress. The factors governing the recruitment of nucleocapsids to the INM are not known. Kinetic analysis of pUL93 demonstrates that this protein is expressed late during infection and localizes primarily to the nucleus of infected cells. pUL93 associates with both virions and capsids and interacts with the components of the nuclear egress complex (NEC), namely, pUL50, pUL53, and pUL97, during infection. Also, multiple regions in pUL93 can independently interact with pUL77, which has been shown to help retain viral DNA during capsid assembly. These studies, combined with our earlier report of an essential role of pUL93 in viral DNA packaging, indicate that pUL93 serves as an important link between nucleocapsid maturation and nuclear egress. IMPORTANCEHCMV causes life-threatening disease and disability in immunocompromised patients and congenitally infected newborns. In this study, we investigated the functions of HCMV essential tegument protein pUL93 and determined that it interacts with the components of the nuclear egress complex, namely, pUL50, pUL53, and pUL97. We also found that pUL93 specifically interacts with pUL77, which helps retain viral DNA during capsid assembly. Together, our data point toward an important role of pUL93 in linking virus maturation to nuclear egress. In addition to expanding our knowledge of the process of HCMV maturation, information from these studies will also be utilized to develop new antiviral therapies. In herpes simplex virus 1 (HSV-1) and pseudorabies virus (PRV), pUL17 and pUL25, the homologs of human cytomegalovirus (HCMV) pUL93 and pUL77, respectively, form a complex called the capsid vertex-specific component (CVSC) (1-5). Five copies of this complex surround each of the 12 vertices of the capsids. This complex was originally termed the C-capsid-specific component (2) based on its presence on DNA containing C-capsids, but it was later found to be present on all capsid types (1). Nevertheless, some differences have been reported in the relative distribution of the CVSC on different capsid types in herpesviruses. In HSV-1, occupancy of the CVSC is approximately 50% on C-capsids and only 25% or less on A-(empty) and B-capsids (scaffold containing), while in PRV, occupancy of the CVSC is nearly 100% on C-capsids and 55% on B-capsids (2, 5). The CVSC is thought to aid in DNA packaging and retention by stabilizing capsids during capsid assembly and in nuclear egress,...

The human cytomegalovirus nuclear egress complex unites multiple functions: Recruitment of effectors, nuclear envelope rearrangement, and docking to nuclear capsids

“…In the case of HCMV, the question has been addressed whether homologous CVSC constituents, pUL77 and pUL93, can interact with one of the core NEC proteins . Very recently, initial evidence was provided that pUL93 may interact with pUL77, pUL50, pUL53, and pUL97 during HCMV infection . Moreover, the viral proteins pUL86 (MCP) and pUL85 were found pUL50‐pUL53‐associated when analyzing complexes from HCMV‐infected cells by mass spectrometry .…”

“…125 Very recently, initial evidence was provided that pUL93 may interact with pUL77, pUL50, pUL53, and pUL97 during HCMV infection. 43,125 Moreover, the viral proteins pUL86 (MCP) and pUL85 were found pUL50-pUL53-associated when analyzing complexes from HCMVinfected cells by mass spectrometry. 59 As both proteins represent capsid components, they may provide a direct link between nuclear viral particles and the NEC.…”

WDR5 Facilitates Human Cytomegalovirus Replication by Promoting Capsid Nuclear Egress

“…This in turn disrupts the nuclear lamina barrier to permit infoldings of the inner nuclear membrane (IINMs) so that capsids can undergo primary envelopment, budding into the perinuclear space. (iii) Enveloped capsids in the perinuclear space then fuse with the outer nuclear membrane (ONM) and undergo deenvelopment to be released into the cytoplasm (14)(15)(16)(17)(18)(19)(20)(21)(22). In the cytoplasm, viral tegument proteins, including pp28, pp65, pp71, pp150, and pUL48, sequentially surround the capsids.…”

Vertex-Specific Proteins pUL17 and pUL25 Mechanically Reinforce Herpes Simplex Virus Capsids

“…Next to HSV-1, similar CVSC complexes are present on purified capsids of the swine alphaherpesvirus pseudorabies virus with even higher occupancy levels (45)(46)(47). Furthermore, homologs of these minor capsid components exist in other alphaherpesviruses, the betaherpesviruses (e.g., pUL77 and pUL93 in human cytomegalovirus) (48), and the gammaherpesviruses (e.g., open reading frame 32 [ORF32] and ORF19 in Kaposi sarcoma-associated virus) (44), suggesting that functional, stabilizing CVSCs are a feature of all herpesviruses (49).…”