Efficient entry of budded virions of Autographa californica multiple nucleopolyhedrovirus into Spodoptera frugiperda cells is dependent on dynamin, Rab5, and Rab11

Coordination of the host Vps4-Vta1 complex and the viral core protein Ac93 facilitates entry of Autographa californica multiple nucleopolyhedrovirus budded virions

The endosomal sorting complex required for transport (ESCRT) is a protein machine mediating membrane scission. In intraluminal vesicle (ILV) formation, ESCRT-0 targets cargoes and recruits ESCRT-I/-II to create membrane invagination, whereas ESCRT-III coordinates with the AAA ATPase Vps4 and its cofactor Vta1 to catalyze the membrane fission. Recently, ESCRT-I/-III and Vps4 were found to be involved in the entry of the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV). However, the necessity of other ESCRT components and the interplay of viral proteins and ESCRTs in regulating the virus entry remain elusive. Here, we identified ESCRT-0 (Hse1 and Vps27), ESCRT-II (Vps22, Vps25, and Vps36), and Vta1 of Spodoptera frugiperda. RNAi depletion of Vta1 but not the components of ESCRT-0 or ESCRT-II in Sf9 cells significantly reduced budded virus (BV) production. Quantitative PCR together with confocal microscopy analyses indicated that Vta1 was required for internalization and endosomal trafficking of BV. In the late phase of infection, although Vps4 and Vta1 were both distributed to the nucleus and at the plasma membrane, depletion of Vta1 did not affect BV release. Further analysis revealed that 7 of 14 BV envelope proteins (Ac75, Ac93, E25, F-like, P33, P48, and vUbiquitin) interacted with Vps4 and Vta1. Intriguingly, Ac93 adopted a similar mode as ESCRT-III proteins to interact with the microtubule-interacting and transport (MIT) domains of Vps4 and Vta1 via its C-terminal MIT-interacting motifs (MIM1), and the interactions were necessary for BV internalization. Together, our studies highlight the coordination of Vps4-Vta1 and Ac93, and probably other BV envelope proteins, in facilitating entry of AcMNPV.IMPORTANCEThe endosomal sorting complex required for transport (ESCRT) system is involved in the entry of diverse DNA and RNA viruses. However, the interplay of viral proteins and ESCRTs in promoting virus endocytosis remains largely unknown. Here, we found that the ESCRT early acting factors ESCRT-0/-II were not necessary for infectious budded virus (BV) production of Autographa californica multiple nucleopolyhedrovirus (AcMNPV). In contrast, the Vps4 cofactor Vta1 was required for entry but not egress of BV. Several core or essential BV envelope proteins were identified to interact with Vps4 and Vta1. Among them, Ac93 plays a central role in connecting other viral proteins and mimics ESCRT-III proteins to interact with Vps4-Vta1, facilitating entry of BV virions. These studies provide evidence for the coordination of viral proteins and ESCRTs in regulating entry of large enveloped DNA viruses.

Multifaceted interactions between host ESCRT-III and budded virus-related proteins involved in entry and egress of the baculovirus Autographa californica multiple nucleopolyhedrovirus

The endosomal sorting complex required for transport (ESCRT) is a conserved protein machine mediating membrane remodeling and scission. In the context of viral infection, different components of the ESCRT-III complex, which serve as the core machinery to catalyze membrane fission, are involved in diverse viruses' entry, replication, and/or budding. However, the interplay between ESCRT-III and viral factors in the virus life cycle, especially for that of large enveloped DNA viruses, is largely unknown. Recently, the ESCRT-III components Vps2B, Vps20, Vps24, Snf7, Vps46, and Vps60 were determined for entry and/or egress of the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV). Here, we identified the final three ESCRT-III components Chm7, Ist1, and Vps2A of Spodoptera frugiperda. Overexpression of the dominant-negative forms of these proteins or RNAi downregulation of their transcripts significantly reduced infectious budded viruses (BVs) production of AcMNPV. Quantitative PCR together with confocal and transmission electron microscopy analysis revealed that these proteins were required for internalization and trafficking of BV during entry and egress of nucleocapsids. In infected Sf9 cells, nine ESCRT-III components were distributed on the nuclear envelope and plasma membrane, and except for Chm7, the other components were also localized to the intranuclear ring zone. Y2H and BiFC analysis revealed that 42 out of 64 BV-related proteins including 35 BV structural proteins and 7 non-BV structural proteins interacted with single or multiple ESCRT-III components. By further mapping the interactome of 64 BV-related proteins, we established the interaction networks of ESCRT-III and the viral protein complexes involved in BV entry and egress.IMPORTANCEFrom archaea to eukaryotes, the endosomal sorting complex required for transport (ESCRT)-III complex is hijacked by many enveloped and nonenveloped DNA or RNA viruses for efficient replication. However, the mechanism of ESCRT-III recruitment, especially for that of large enveloped DNA viruses, remains elusive. Recently, we found the ESCRT-III components Vps2B, Vps20, Vps24, Snf7, Vps46, and Vps60 are necessary for the entry and/or egress of budded viruses (BVs) of Autographa californica multiple nucleopolyhedrovirus. Here, we demonstrated that the other three ESCRT-III components Chm7, Ist1, and Vps2A play similar roles in BV infection. By determining the subcellular localization of ESCRT-III components in infected cells and mapping the interaction of nine ESCRT-III components and 64 BV-related proteins, we built the interaction networks of ESCRT-III and the viral protein complexes involved in BV entry and egress. These studies provide a fundamental basis for understanding the mechanism of the ESCRT-mediated membrane remodeling for replication of baculoviruses.

Label-free proteomics analysis on the envelope of budded viruses of Bombyx mori nucleopolyhedrovirus harboring differential localized GP64

Bombyx mori nucleopolyhedrovirus (BmNPV) GP64 is the key membrane fusion protein that mediates budded virus (BV) infection. We recently reported that BmNPV GP64's n-region of signal peptide (SP) blocked the SP-cleavage and mediated GP64 localization on the plasma membrane (PM); n-region (SP^∆nGP64) absence caused GP64 intracellular localization, however, SP^∆nGP64 was still incorporated into virion to generate BVs with lower infectivity. To better understand the biogenesis of the envelope of BmNPV BV, we conducted a label-free ESI mass spectrometry analysis of the envelope of purified BVs harboring PM localized GP64 or intracellular localized SP^∆nGP64. The results indicated that 31 viral proteins were identified on the envelope, among which 15 were reported in other viruses. The other 16 proteins were first reported in BmNPV BV, including the BmNPV-specific protein BRO-A and proteins associated with vesicle transportation. Six proteins with significant intensity differences were detected in virions with differential localized GP64, and five specific proteins were identified in virions with GP64. Meanwhile, we identified 81 host proteins on the envelope, and seven lipoproteins were first identified in baculovirus virion; other 74 proteins are involved in the cytoskeleton, DNA-binding, vesicle transport, etc. In the meantime, eight and five specific host proteins were, respectively, identified in GP64 and SP^∆nGP64's virions. The two virions shared 68 common host proteins, and 8 proteins were identified on their envelopes with a significant difference. This study provides new insight into the protein composition of BmNPV BV and a clue for further investigation of the budding mechanism of BmNPV.

Functional Analysis of a Frontal miRNA Cluster Located in the Large Latency Transcript of Pseudorabies Virus

MicroRNAs (miRNAs) have been identified as a class of crucial regulators of virus-host crosstalk, modulating such processes as viral replication, antiviral immune response, viral latency, and pathogenesis. Pseudorabies virus (PRV), a model for the study of alphaherpesvirus biology, codes for 11 distinct miRNAs mapped to the ~4.6 kb intron of Large Latency Transcript (LLT). Recent studies have revealed the role of clusters consisting of nine and eleven miRNA genes in the replication and virulence of PRV. The function of separate miRNA species in regulating PRV biology has not been thoroughly investigated. To analyze the regulatory potential of three PRV miRNAs located in the frontal cluster of the LLT intron, we generated a research model based on the constitutive expression of viral miRNAs in swine testis cells (ST_LLT [1–3] cell line). Using a cell culture system providing a stable production of individual miRNAs at high levels, we demonstrated that the LLT [1–3] miRNA cluster significantly downregulated IE180, EP0, and gE at the early stages of PRV infection. It was further determined that LLT [1–3] miRNAs could regulate the infection process, leading to a slight distortion in transmission and proliferation ability. Collectively, our findings indicate the potential of LLT [1–3] miRNAs to retard the host responses by reducing viral antigenic load and suppressing the expansion of progeny viruses at the early stages of infection.

Critical Residues and Contacts within Domain IV of Autographa californica Multiple Nucleopolyhedrovirus GP64 Contribute to Its Refolding during Membrane Fusion

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) GP64 is a class III viral fusion protein that mediates low-pH-triggered membrane fusion during virus entry. Although the structure of GP64 in a postfusion conformation has been solved, its prefusion structure and the mechanism of how the protein refolds to execute fusion are unknown. In its postfusion structure, GP64 is composed of five domains (domains I to V). Domain IV (amino acids [aa] 374 to 407) contains two loops (loop 1 and loop 2) that form a hydrophobic pocket at the membrane-distal end of the molecule. To determine the roles of domain IV, we used alanine-scanning mutagenesis to replace each of the individual residues and the contact-forming residues within domain IV and evaluate their contributions to GP64-mediated membrane fusion and virus infection. In many cases, replacement of a single amino acid had no significant impact on GP64. However, replacement of R392 or disruption of the N381-N385, N384-Y388, N385-W393, or K389-W393 contact resulted in poor cell surface expression and fusion loss of the modified GP64, whereas replacement of E390 or G391 or disruption of the N381-K389, N381-Q401, or N381-I403 contact reduced the cell surface expression level of the constructs and the ability of GP64 to mediate fusion pore expansion. In contrast, replacement of N407 or disruption of contact D404-S406 appeared to restrict fusion pore expansion without affecting expression. Combined with the finding that these constructs remain in the prefusion conformation or have a dramatically less efficient transition from the prefusion to the postfusion state under acidic conditions, we proposed that domain IV is necessary for refolding of GP64 during membrane fusion.IMPORTANCE Baculovirus GP64 is grouped with rhabdovirus G, herpesvirus gB, and thogotovirus glycoproteins as a class III viral fusion protein. In their postfusion structures, these proteins contain five domains (domains I to V). Distinct from domain IV of rhabdovirus G and herpesvirus gB proteins, which is composed of β-sheets, domain IV of GP64 is a loop region; the same domain in thogotovirus glycoproteins has not been solved. In addition, domain IV is proximal to domain I (fusion domain) in prefusion structures of vesicular stomatitis virus (VSV) G and human cytomegalovirus (HCMV) gB but resides at the domain I-distal end of the molecule in a postfusion conformation. In this study, we identified that highly conserved residues and contacts within domain IV of AcMNPV GP64 are necessary for low-pH-triggered conformational change and fusion pore expansion. Our results highlight the roles of domain IV of class III viral fusion proteins in refolding during membrane fusion.