Nuclear F-actin is required for AcMNPV nucleocapsid morphogenesis

Nuclear actin filaments - a historical perspective

The view on nuclear filaments formed by non-skeletal β-actin has significantly changed over the decades. Initially, filamentous actin was observed in amphibian oocyte nuclei and only under specific cell stress conditions in mammalian cell nuclei. Improved labeling and imaging technologies have permitted insights into a transient but microscopically apparent filament network that is relevant for chromatin organization, biomechanics of the mammalian cell nucleus, gene expression, and DNA damage repair. Here, we will provide a historical perspective on the developing insight into nuclear actin filaments.

Baculovirus Actin Rearrangement-Inducing Factor 1 Can Remodel the Mammalian Actin Cytoskeleton

The actin rearrangement-inducing factor 1 (Arif-1) of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is an early viral protein that manipulates the actin cytoskeleton of host insect cells. Arif-1 is conserved among alphabaculoviruses and is responsible for the accumulation of F-actin at the plasma membrane during the early phase of infection. However, the molecular mechanism underlying Arif-1-induced cortical actin accumulation is still open. Recent studies have demonstrated the formation of invadosome-like structures induced by Arif-1, suggesting a function in systemic virus spread. Here, we addressed whether Arif-1 is able to manipulate the actin cytoskeleton of mammalian cells comparably to insect cells. Strikingly, transient overexpression of Arif-1 in B16-F1 mouse melanoma cells revealed pronounced F-actin remodeling. Actin assembly was increased, and intense membrane ruffling occurred at the expense of substrate-associated lamellipodia. Deletion mutagenesis studies of Arif-1 confirmed that the C-terminal cytoplasmic region was not sufficient to induce F-actin remodeling, supporting that the transmembrane region for Arif-1 function is also required in mammalian cells. The similarities between Arif-1-induced actin remodeling in insect and mammalian cells indicate that Arif-1 function relies on conserved cellular interaction partners and signal transduction pathways, thus providing an experimental tool to elucidate the underlying mechanism. IMPORTANCE Virus-induced changes of the host cell cytoskeleton play a pivotal role in the pathogenesis of viral infections. The baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is known for intervening with the regulation of the host actin cytoskeleton in a wide manner throughout the infection cycle. The actin rearrangement-inducing factor 1 (Arif-1) is a viral protein that causes actin rearrangement during the early phase of AcMNPV infection. Here, we performed overexpression studies of Arif-1 in mammalian cells to establish an experimental tool that allows elucidation of the mechanism underlying the Arif-1-induced remodeling of actin dynamics in a well-characterized and genetically accessible system.

Nuclear Cytoskeleton in Virus Infection

The nuclear lamina is the main component of the nuclear cytoskeleton that maintains the integrity of the nucleus. However, it represents a natural barrier for viruses replicating in the cell nucleus. The lamina blocks viruses from being trafficked to the nucleus for replication, but it also impedes the nuclear egress of the progeny of viral particles. Thus, viruses have evolved mechanisms to overcome this obstacle. Large viruses induce the assembly of multiprotein complexes that are anchored to the inner nuclear membrane. Important components of these complexes are the viral and cellular kinases phosphorylating the lamina and promoting its disaggregation, therefore allowing virus egress. Small viruses also use cellular kinases to induce lamina phosphorylation and the subsequent disruption in order to facilitate the import of viral particles during the early stages of infection or during their nuclear egress. Another component of the nuclear cytoskeleton, nuclear actin, is exploited by viruses for the intranuclear movement of their particles from the replication sites to the nuclear periphery. This study focuses on exploitation of the nuclear cytoskeleton by viruses, although this is just the beginning for many viruses, and promises to reveal the mechanisms and dynamic of physiological and pathological processes in the nucleus.

Baculovirus actin-rearrangement-inducing factor ARIF-1 induces the formation of dynamic invadosome clusters

The baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV), a pathogen of lepidopteran insects, has a striking dependence on the host cell actin cytoskeleton. During the delayed-early stage of infection, AcMNPV was shown to induce the accumulation of actin at the cortex of infected cells. However, the dynamics and molecular mechanism of cortical actin assembly remained unknown. Here, we show that AcMNPV induces dynamic cortical clusters of dot-like actin structures that mediate degradation of the underlying extracellular matrix and therefore function similarly to clusters of invadosomes in mammalian cells. Furthermore, we find that the AcMNPV protein actin-rearrangement-inducing factor-1 (ARIF-1), which was previously shown to be necessary and sufficient for cortical actin assembly and efficient viral infection in insect hosts, is both necessary and sufficient for invadosome formation. We mapped the sequences within the C-terminal cytoplasmic region of ARIF-1 that are required for invadosome formation and identified individual tyrosine and proline residues that are required for organizing these structures. Additionally, we found that ARIF-1 and the invadosome-associated proteins cortactin and the Arp2/3 complex localize to invadosomes and Arp2/3 complex is required for their formation. These ARIF-1-induced invadosomes may be important for the function of ARIF-1 in systemic virus spread.

Baculovirus Expression and Functional Analysis of Vpa2 Proteins from Bacillus thuringiensis

The mode of action underlying the insecticidal activity of the Bacillus thuringiensis (Bt) binary pesticidal protein Vpa1/Vpa2 is uncertain. In this study, three recombinant baculoviruses were constructed using Bac-to-Bac technology to express Vpa2Ac1 and two novel Vpa2-like genes, Vpa2-like1 and Vpa2-like2, under the baculovirus p10 promoter in transfected Sf9 cells. Pairwise amino acid analyses revealed a higher percentage of identity and a lower number of gaps between Vpa2Ac1 and Vpa2-like2 than to Vpa2-like1. Moreover, Vpa2-like1 lacked the conserved Ser-Thr-Ser motif, involved in NAD binding, and the (F/Y)xx(Q/E)xE consensus sequence, characteristic of the ARTT toxin family involved in actin polymerization. Vpa2Ac1, Vpa2-like1 and Vpa2-like2 transcripts and proteins were detected in Sf9 culture cells, but the signals of Vpa2Ac1 and Vpa2-like2 were weak and decreased over time. Sf9 cells infected by a recombinant bacmid expressing Vpa2-like1 showed typical circular morphology and produced viral occlusion bodies (OBs) at the same level as the control virus. However, expression of Vpa2Ac1 and Vpa2-like2 induced cell polarization, similar to that produced by the microfilament-destabilizing agent cytochalasin D and OBs were not produced. The presence of filament disrupting agents, such as nicotinamide and nocodazole, during transfection prevented cell polarization and OB production was observed. We conclude that Vpa2Ac1 and Vpa2-like2 proteins likely possess ADP-ribosyltransferase activity that modulated actin polarization, whereas Vpa2-like1 is not a typical Vpa2 protein. Vpa2-like2 has now been designated Vpa2Ca1 (accession number AAO86513) by the Bacillus thuringiensis delta-endotoxin nomenclature committee.

Baculovirus Actin-Based Motility Drives Nuclear Envelope Disruption and Nuclear Egress

Viruses that replicate in the host cell nucleus face challenges in usurping cellular pathways to enable passage through the nuclear envelope [1]. Baculoviruses are enveloped, double-stranded DNA viruses that infect lepidopteran insects and are tools for protein expression, cell transduction, and pest management [2-4]. The type species Autographa californica M nucleopolyhedrovirus (AcMNPV) shares with other pathogens an ability to assemble host actin monomers (G-actin) into actin filaments (F-actin) to drive motility [5]. During early infection, actin-based motility in the cytoplasm speeds AcMNPV transit to the nucleus and passage through nuclear pores, enabling nuclear ingress [6, 7]. During late infection, AcMNPV assembles F-actin within the nucleus [8], which is essential for virus production [9, 10]. However, the function of nuclear F-actin is poorly understood [11], and its mechanistic role in AcMNPV infection was unknown. We show that AcMNPV mobilizes actin within the nucleus to promote egress. AcMNPV nucleocapsids exhibit intranuclear actin-based motility, mediated by the viral protein P78/83 and the host Arp2/3 complex. Viral motility drives transit to the nuclear periphery and is required for viruses to enter protrusions of the nuclear envelope. Moreover, actin polymerization is necessary for viral disruption of nuclear envelope integrity during egress. In the cytoplasm, viruses use actin-based motility to reach the plasma membrane to enable budding. Our results demonstrate that pathogens can harness actin polymerization to disrupt the nuclear envelope. Employing actin for nuclear envelope disruption may reflect viral appropriation of normal functions of nuclear actin in nuclear envelope integrity, stability, and remodeling.

Ac102 Participates in Nuclear Actin Polymerization by Modulating BV/ODV-C42 Ubiquitination during Autographa californica Multiple Nucleopolyhedrovirus Infection

As a virus-encoded actin nucleation promoting factor (NPF), P78/83 induces actin polymerization to assist in Autographa californica multiple nucleopolyhedrovirus (AcMNPV) propagation. According to our previous study, although P78/83 actively undergoes ubiquitin-independent proteasomal degradation, AcMNPV encodes budded virus/occlusion derived virus (BV/ODV)-C42 (C42), which allows P78/83 to function as a stable NPF by inhibiting its degradation during viral infection. However, whether there are other viral proteins involved in regulating P78/83-induced actin polymerization has yet to be determined. In this study, we found that Ac102, an essential viral gene product previously reported to play a key role in mediating the nuclear accumulation of actin during AcMNPV infection, is a novel regulator of P78/83-induced actin polymerization. By characterizing an ac102 knockout bacmid, we demonstrated that Ac102 participates in regulating nuclear actin polymerization as well as the morphogenesis and distribution of capsid structures in the nucleus. These regulatory effects are heavily dependent on an interaction between Ac102 and C42. Further investigation revealed that Ac102 binds to C42 to suppress K48-linked ubiquitination of C42, which decreases C42 proteasomal degradation and consequently allows P78/83 to function as a stable NPF to induce actin polymerization. Thus, Ac102 and C42 form a regulatory cascade to control viral NPF activity, representing a sophisticated mechanism for AcMNPV to orchestrate actin polymerization in both a ubiquitin-dependent and ubiquitin-independent manner.IMPORTANCE Actin is one of the most functionally important proteins in eukaryotic cells. Morphologically, actin can be found in two forms: a monomeric form called globular actin (G-actin) and a polymeric form called filamentous actin (F-actin). G-actin can polymerize to form F-actin, and nucleation promoting factor (NPF) is the initiator of this process. Many viral pathogens harness the host actin polymerization machinery to assist in virus propagation. Autographa californica multiple nucleopolyhedrovirus (AcMNPV) induces actin polymerization in host cells. P78/83, a viral NPF, is responsible for this process. Previously, we identified that BV/ODV-C42 (C42) binds to P78/83 and protects it from degradation. In this report, we determined that another viral protein, Ac102, is involved in modulating C42 ubiquitination and, consequently, ensures P78/83 activity as an NPF to initiate actin polymerization. This regulatory cascade represents a novel mechanism by which a virus can harness the cellular actin cytoskeleton to assist in viral propagation.

Baculovirus AC102 Is a Nucleocapsid Protein That Is Crucial for Nuclear Actin Polymerization and Nucleocapsid Morphogenesis

The baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV), the type species of alphabaculoviruses, is an enveloped DNA virus that infects lepidopteran insects and is commonly known as a vector for protein expression and cell transduction. AcMNPV belongs to a diverse group of viral and bacterial pathogens that target the host cell actin cytoskeleton during infection. AcMNPV is unusual, however, in that it absolutely requires actin translocation into the nucleus early in infection and actin polymerization within the nucleus late in infection coincident with viral replication. Of the six viral factors that are sufficient, when coexpressed, to induce the nuclear localization of actin, only AC102 is essential for viral replication and the nuclear accumulation of actin. We therefore sought to better understand the role of AC102 in actin mobilization in the nucleus early and late in infection. Although AC102 was proposed to function early in infection, we found that AC102 is predominantly expressed as a late protein. In addition, we observed that AC102 is required for F-actin assembly in the nucleus during late infection, as well as for proper formation of viral replication structures and nucleocapsid morphogenesis. Finally, we found that AC102 is a nucleocapsid protein and a newly recognized member of a complex consisting of the viral proteins EC27, C42, and the actin polymerization protein P78/83. Taken together, our findings suggest that AC102 is necessary for nucleocapsid morphogenesis and actin assembly during late infection through its role as a component of the P78/83-C42-EC27-AC102 protein complex.IMPORTANCE The baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is an important biotechnological tool for protein expression and cell transduction, and related nucleopolyhedroviruses are also used as environmentally benign insecticides. One impact of our work is to better understand the fundamental mechanisms through which AcMNPV exploits the cellular machinery of the host for replication, which may aid in the development of improved baculovirus-based research and industrial tools. Moreover, AcMNPV's ability to mobilize the host actin cytoskeleton within the cell's nucleus during infection makes it a powerful cell biological tool. It is becoming increasingly clear that actin plays important roles in the cell's nucleus, and yet the regulation and function of nuclear actin is poorly understood. Our work to better understand how AcMNPV relocalizes and polymerizes actin within the nucleus may reveal fundamental mechanisms that govern nuclear actin regulation and function, even in the absence of viral infection.

The role of viral protein Ac34 in nuclear relocation of subunits of the actin-related protein 2/3 complex

The actin nucleator actin-related protein complex (Arp2/3) is composed of seven subunits: Arp2, Arp3, p40/ARPC1 (P40), p34/ARPC2 (P34), p21/ARPC3 (P21), p20/ARPC4 (P20), and p16/ARPC5 (P16). Arp2/3 plays crucial roles in a variety of cellular activities through regulation of actin polymerization. Autographa californica multiple nucleopolyhedrovirus (AcMNPV), one of the beststudied alphabaculoviruses, induces Arp2/3 nuclear relocation and mediates nuclear actin polymerization to assist in virus replication. We have demonstrated that Ac34, a viral late-gene product, induces translocation of the P40 subunit of Arp2/3 to the nucleus during AcMNPV infection. However, it remains unknown whether Ac34 could relocate other Arp2/3 subunits to the nucleus. In this study, the effects of the viral protein Ac34 on the distribution of these subunits were studied by an immunofluorescence assay. Arp2, P34, P21, and P20 cloned from Spodoptera frugiperda (Sf9) cells showed mainly cytoplasmic localization and were relocated to the nucleus in the presence of Ac34. In addition, Arp3 was localized in the cytoplasm in both the presence and absence of Ac34, and P16 showed whole-cell localization. In contrast to Sf9 cells, all subunits of mammalian Arp2/3 showed no nuclear relocation in the presence of Ac34. Co-immunoprecipitation analysis of the interaction between Ac34 and Arp2/3 subunits revealed that Ac34 bound to P40, P34, and P20 of Sf9 cells. However, none of the subunits of mammalian Arp2/3 interacted with Ac34, indicating that protein-protein interaction is essential for Ac34 to relocate Arp2/3 subunits to the nucleus.

Autographa californica Multiple Nucleopolyhedrovirus Ac34 Protein Retains Cellular Actin-Related Protein 2/3 Complex in the Nucleus by Subversion of CRM1-Dependent Nuclear Export

Actin, nucleation-promoting factors (NPFs), and the actin-related protein 2/3 complex (Arp2/3) are key elements of the cellular actin polymerization machinery. With nuclear actin polymerization implicated in ever-expanding biological processes and the discovery of the nuclear import mechanisms of actin and NPFs, determining Arp2/3 nucleo-cytoplasmic shuttling mechanism is important for understanding the function of nuclear actin. A unique feature of alphabaculovirus infection of insect cells is the robust nuclear accumulation of Arp2/3, which induces actin polymerization in the nucleus to assist in virus replication. We found that Ac34, a viral late gene product encoded by the alphabaculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV), is involved in Arp2/3 nuclear accumulation during virus infection. Further assays revealed that the subcellular distribution of Arp2/3 under steady-state conditions is controlled by chromosomal maintenance 1 (CRM1)-dependent nuclear export. Upon AcMNPV infection, Ac34 inhibits CRM1 pathway and leads to Arp2/3 retention in the nucleus.

Viruses That Exploit Actin-Based Motility for Their Replication and Spread

The actin cytoskeleton is a crucial part of the eukaryotic cell. Viruses depend on host cells for their replication, and, as a result, many have developed ways of manipulating the actin network to promote their spread. This chapter reviews the various ways in which viruses utilize the actin cytoskeleton at discrete steps in their life cycle, from entry into the host cell, replication, and assembly of new progeny to virus release. Various actin inhibitors that function in different ways to affect proper actin dynamics can be used to parse the role of actin at these steps.

The Autographa californica Multiple Nucleopolyhedrovirus ac54 Gene Is Crucial for Localization of the Major Capsid Protein VP39 at the Site of Nucleocapsid Assembly

Baculovirus DNAs are synthesized and inserted into preformed capsids to form nucleocapsids at a site in the infected cell nucleus, termed the virogenic stroma. Nucleocapsid assembly ofAutographa californicamultiple nucleopolyhedrovirus (AcMNPV) requires the major capsid protein VP39 and nine minor capsid proteins, including VP1054. However, how VP1054 participates in nucleocapsid assembly remains elusive. In this study, the VP1054-encoding gene (ac54) was deleted to generate theac54-knockout AcMNPV (vAc54KO). In vAc54KO-transfected cells, nucleocapsid assembly was disrupted, leading to the formation of abnormally elongated capsid structures. Interestingly, unlike cells transfected with AcMNPV mutants lacking other minor capsid proteins, in which capsid structures were distributed within the virogenic stroma,ac54ablation resulted in a distinctive location of capsid structures and VP39 at the periphery of the nucleus. The altered distribution pattern of capsid structures was also observed in cells transfected with AcMNPV lacking BV/ODV-C42 or in cytochalasind-treated AcMNPV-infected cells. BV/ODV-C42, along with PP78/83, has been shown to promote nuclear filamentous actin (F-actin) formation, which is another requisite for nucleocapsid assembly. Immunofluorescence using phalloidin indicated that the formation and distribution of nuclear F-actin were not affected byac54deletion. However, immunoelectron microscopy revealed that BV/ODV-C42, PP78/83, and 38K failed to integrate into capsid structures in the absence of VP1054, and immunoprecipitation further demonstrated that in transient expression assays, VP1054 interacted with BV/ODV-C42 and VP80 but not VP39. Our findings suggest that VP1054 plays an important role in the transport of capsid proteins to the nucleocapsid assembly site prior to the process of nucleocapsid assembly.

IMPORTANCE

Baculoviruses are large DNA viruses whose replication occurs within the host nucleus. The localization of capsids into the capsid assembly site requires virus-induced nuclear F-actin; the inhibition of nuclear F-actin formation results in the retention of capsid structures at the periphery of the nucleus. In this paper, we note that the minor capsid protein VP1054 is essential for the localization of capsid structures, the major capsid protein VP39, and the minor capsid protein 38K into the capsid assembly site. Moreover, VP1054 is crucial for correct targeting of the nuclear F-actin factors BV/ODV-C42 and PP78/83 for capsid maturation. However, the formation and distribution of nuclear F-actin are not affected by the lack of VP1054. We further reveal that VP1054 interacts with BV/ODV-C42 and a capsid transport-related protein, VP80. Taken together, our findings suggest that VP1054 plays a unique role in the pathway(s) for transport of capsid proteins.

Trichoplusia ni Kinesin-1 Associates with Autographa californica Multiple Nucleopolyhedrovirus Nucleocapsid Proteins and Is Required for Production of Budded Virus

The mechanism by which nucleocapsids of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) egress from the nucleus to the plasma membrane, leading to the formation of budded virus (BV), is not known. AC141 is a nucleocapsid-associated protein required for BV egress and has previously been shown to be associated with β-tubulin. In addition, AC141 and VP39 were previously shown by fluorescence resonance energy transfer by fluorescence lifetime imaging to interact directly with the Drosophila melanogaster kinesin-1 light chain (KLC) tetratricopeptide repeat (TPR) domain. These results suggested that microtubule transport systems may be involved in baculovirus nucleocapsid egress and BV formation. In this study, we investigated the role of lepidopteran microtubule transport using coimmunoprecipitation, colocalization, yeast two-hybrid, and small interfering RNA (siRNA) analyses. We show that nucleocapsid AC141 associates with the lepidopteran Trichoplusia ni KLC and kinesin-1 heavy chain (KHC) by coimmunoprecipitation and colocalization. Kinesin-1, AC141, and microtubules colocalized predominantly at the plasma membrane. In addition, the nucleocapsid proteins VP39, FP25, and BV/ODV-C42 were also coimmunoprecipitated with T. ni KLC. Direct analysis of the role of T. ni kinesin-1 by downregulation of KLC by siRNA resulted in a significant decrease in BV production. Nucleocapsids labeled with VP39 fused with three copies of the mCherry fluorescent protein also colocalized with microtubules. Yeast two-hybrid analysis showed no evidence of a direct interaction between kinesin-1 and AC141 or VP39, suggesting that either other nucleocapsid proteins or adaptor proteins may be required. These results further support the conclusion that microtubule transport is required for AcMNPV BV formation.

IMPORTANCE

In two key processes of the replication cycle of the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV), nucleocapsids are transported through the cell. These include (i) entry of budded virus (BV) into the host cell and (ii) egress and budding of nucleocapsids newly produced from the plasma membrane. Prior studies have shown that the entry of nucleocapsids involves the polymerization of actin to propel nucleocapsids to nuclear pores and entry into the nucleus. For the spread of infection, progeny viruses must rapidly exit the infected cells, but the mechanism by which AcMNPV nucleocapsids traverse the cytoplasm is unknown. In this study, we examined whether nucleocapsids interact with lepidopteran kinesin-1 motor molecules and are potentially carried as cargo on microtubules to the plasma membrane in AcMNPV-infected cells. This study indicates that microtubule transport is utilized for the production of budded virus.

Viruses that ride on the coat-tails of actin nucleation

Actin nucleation drives a diversity of critical cellular processes and the motility of a select group of viral pathogens. Vaccinia virus and baculovirus, Autographa californica multiple nucleopolyhedrovirus, recruit and activate the cellular actin nucleator, the Arp2/3 complex, at the surface of virus particles thereby instigating highly localized actin nucleation. The extension of these filaments provides a mechanical force that bestows the ability to navigate the intracellular environment and promote their infectious cycles. This review outlines the viral and cellular proteins that initiate and regulate the signalling networks leading to viral modification of the actin cytoskeleton and summarizes recent insights into the role of actin-based virus transport.

Effects of recombinant baculovirus AcMNPV-BmK IT on the formation of early cables and nuclear polymerization of actin in Sf9 cells

Autographa californica nuclearpoly hedrosis virus (AcMNPV) is one of the most important baculoviridae. However, the application of AcMNPV as a biocontrol agent has been limited. Previously, we engineered Buthus martensii Karsch insect toxin (BmK IT) gene into the genome of AcMNPV. The bioassay data indicated that the recombinant baculovirus AcMNPV-BmK IT significantly enhanced the anti-insect efficacy of the virus. The actin cytoskeleton is the major component beneath the surface of eukaryotic cells. In this report, the effects of AcMNPV-BmK IT on the formation of early cables of actin and nuclear filamentous-actin (F-actin) were studied. The results indicated that these baculovirus induced rearrangement of the actin cytoskeleton of host cells during infection and actin might participate in the transportation of baculovirus from cytoplasm to the nuclei. AcMNPV-BmK IT delayed the formation of early cables of actin and nuclear F-actin and accelerated the clearance of actin in the nuclei.

Identification of a novel regulatory sequence of actin nucleation promoting factor encoded by Autographa californica multiple nucleopolyhedrovirus

Actin polymerization induced by nucleation promoting factors (NPFs) is one of the most fundamental biological processes in eukaryotic cells. NPFs contain a conserved output domain (VCA domain) near the C terminus, which interacts with and activates the cellular actin-related protein 2/3 complex (Arp2/3) to induce actin polymerization and a diverse regulatory domain near the N terminus. Autographa californica multiple nucleopolyhedrovirus (AcMNPV) nucleocapsid protein P78/83 is a virus-encoded NPF that contains a C-terminal VCA domain and induces actin polymerization in virus-infected cells. However, there is no similarity between the N terminus of P78/83 and that of other identified NPFs, suggesting that P78/83 may possess a unique regulatory mechanism. In this study, we identified a multifunctional regulatory sequence (MRS) located near the N terminus of P78/83 and determined that one of its functions is to serve as a degron to mediate P78/83 degradation in a proteasome-dependent manner. In AcMNPV-infected cells, the MRS also binds to another nucleocapsid protein, BV/ODV-C42, which stabilizes P78/83 and modulates the P78/83-Arp2/3 interaction to orchestrate actin polymerization. In addition, the MRS is also essential for the incorporation of P78/83 into the nucleocapsid, ensuring virion mobility powered by P78/83-induced actin polymerization. The triple functions of the MRS enable P78/83 to serve as an essential viral protein in the AcMNPV replication cycle, and the possible roles of the MRS in orchestrating the virus-induced actin polymerization and viral genome decapsidation are discussed.

Viral exploitation of actin: force-generation and scaffolding functions in viral infection

As a fundamental component of the host cellular cytoskeleton, actin is routinely engaged by infecting viruses. Furthermore, viruses from diverse groups, and infecting diverse hosts, have convergently evolved an array of mechanisms for manipulating the actin cytoskeleton for efficacious infection. An ongoing chorus of research now indicates that the actin cytoskeleton is critical for viral replication at many stages of the viral life cycle, including binding, entry, nuclear localization, genomic transcription and reverse transcription, assembly, and egress/dissemination. Specifically, viruses subvert the force-generating and macromolecular scaffolding properties of the actin cytoskeleton to propel viral surfing, internalization, and migration within the cell. Additionally, viruses utilize the actin cytoskeleton to support and organize assembly sites, and eject budding virions for cell-to-cell transmission. It is the purpose of this review to provide an overview of current research, focusing on the various mechanisms and themes of virus-mediated actin modulation described therein.

Arp2/3-mediated actin-based motility: a tail of pathogen abuse

Intracellular pathogens have developed elaborate mechanisms to exploit the different cellular systems of their unwilling hosts to facilitate their entry, replication, and survival. In particular, a diverse range of bacteria and viruses have evolved unique strategies to harness the power of Arp2/3-mediated actin polymerization to enhance their cell-to-cell spread. In this review, we discuss how studying these pathogens has revolutionized our molecular understanding of Arp2/3-dependent actin assembly and revealed key signaling pathways regulating actin assembly in cells. Future analyses of microbe-host interactions are likely to continue uncovering new mechanisms regulating actin assembly and dynamics, as well as unexpected cellular functions for actin. Further, studies with known and newly emerging pathogens will also undoubtedly continue to enhance our understanding of the role of the actin cytoskeleton during pathogenesis and potentially highlight future therapeutic approaches.

Toward system-level understanding of baculovirus-host cell interactions: from molecular fundamental studies to large-scale proteomics approaches

Baculoviruses are insect viruses extensively exploited as eukaryotic protein expression vectors. Molecular biology studies have provided exciting discoveries on virus-host interactions, but the application of omic high-throughput techniques on the baculovirus-insect cell system has been hampered by the lack of host genome sequencing. While a broader, systems-level analysis of biological responses to infection is urgently needed, recent advances on proteomic studies have yielded new insights on the impact of infection on the host cell. These works are reviewed and critically assessed in the light of current biological knowledge of the molecular biology of baculoviruses and insect cells.

Nuclear localization of actin requires AC102 in Autographa californica multiple nucleopolyhedrovirus-infected cells

Autographa californica multiple nucleopolyhedrovirus requires nuclear actin for progeny virus production and thereby encodes viral products that ensure actin's translocation to and retention within the nucleus. Current evidence suggests that the ie0-ie1 gene complex along with five nuclear localization of actin (NLA) genes are sufficient for NLA in transient transfection experiments. Here we report that, during infection, only one of the five NLA genes, Ac102, was essential for NLA, and that AC102 had at least one other activity critical for budded virus (BV) production. Viral deletion mutants in the other four NLA genes were viable, with only two having replication phenotypes different from that of the wild type. Infection with AcΔpe38 revealed a delay in both BV production and NLA. Infection with AcΔ152 revealed a delay in BV production, but no corresponding delay in NLA. Infection with either AcΔpe38 or AcΔ152 resulted in slightly reduced BV titres. Deletion of Ac004 or he65 had no impact on actin translocation kinetics, timing of BV production or BV titres. These results implicate AC102 as a key player in baculovirus manipulation of actin.

Nuclear actin and lamins in viral infections

Lamins are the best characterized cytoskeletal components of the cell nucleus that help to maintain the nuclear shape and participate in diverse nuclear processes including replication or transcription. Nuclear actin is now widely accepted to be another cytoskeletal protein present in the nucleus that fulfills important functions in the gene expression. Some viruses replicating in the nucleus evolved the ability to interact with and probably utilize nuclear actin for their replication, e.g., for the assembly and transport of capsids or mRNA export. On the other hand, lamins play a role in the propagation of other viruses since nuclear lamina may represent a barrier for virions entering or escaping the nucleus. This review will summarize the current knowledge about the roles of nuclear actin and lamins in viral infections.

Essential C-terminal region of the baculovirus minor capsid protein VP80 binds DNA

The essential Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) minor capsid protein VP80 has been recently shown to interact with the virus-triggered, nuclear F-actin cytoskeleton. A role for VP80 in virus morphogenesis has been proposed in the maturation of progeny nucleocapsids and in their egress from the virogenic stroma toward the nuclear periphery by a mechanism, which also includes F-actin filaments. We performed functional mapping of VP80 demonstrating that its highly conserved C-terminal region plays a crucial role in virion morphogenesis. Protein database mining identified a putative basic helix-loop-helix (bHLH) domain, a DNA-binding module typical for eukaryotic transcription factors, in the essential C-terminal region of VP80. Using a molecular modeling approach, we predicted the three-dimensional structure of this domain, revealing some unique properties. Biochemical assays proved that VP80 can form homodimers, a critical prerequisite of DNA-binding bHLH proteins. The ability of VP80 to bind DNA was subsequently confirmed by an electrophoretic mobility shift assay. We further show that AcMNPV DNA replication occurs in the absence of VP80. Immunolabeling of VP80 in baculovirus-infected cells rather points toward its involvement in nucleocapsid maturation. The competence of VP80 to interact with both F-actin and DNA provides novel insight into baculovirus morphogenesis.

Nuclear transport of baculovirus: revealing the nuclear pore complex passage

Baculoviruses are one of the largest viruses that replicate in the nucleus of their host cells. During an infection the capsid, containing the DNA viral genome, is released into the cytoplasm and delivers the genome into the nucleus by a mechanism that is largely unknown. Here, we used capsids of the baculovirus Autographa californica multiple nucleopolyhedrovirus in combination with electron microscopy and discovered this capsid crosses the NPC and enters into the nucleus intact, where it releases its genome. To better illustrate the existence of this capsid through the NPC in its native conformation, we reconstructed the nuclear import event using electron tomography. In addition, using different experimental conditions, we were able to visualize the intact capsid interacting with NPC cytoplasmic filaments, as an initial docking site, and midway through the NPC. Our data suggests the NPC central channel undergoes large-scale rearrangements to allow translocation of the intact 250-nm long baculovirus capsid. We discuss our results in the light of the hypothetical models of NPC function.

Baculovirus VP80 protein and the F-actin cytoskeleton interact and connect the viral replication factory with the nuclear periphery

Recently, we showed that the Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) VP80 protein is essential for the formation of both virion types, budded virus (BV) and occlusion-derived virus (ODV). Deletion of the vp80 gene did not affect assembly of nucleocapsids. However, these nucleocapsids were not able to migrate from the virogenic stroma to the nuclear periphery. In the current paper, we constructed a baculovirus recombinant with enhanced-green fluorescent protein (EGFP)-tagged VP80, allowing visualization of the VP80 distribution pattern during infection. In baculovirus-infected cells, the EGFP-VP80 protein is entirely localized in nuclei, adjacent to the virus-triggered F-actin scaffold that forms a highly organized three-dimensional network connecting the virogenic stroma physically with the nuclear envelope. Interaction between VP80 and host actin was confirmed by coimmunoprecipitation. We further showed that VP80 is associated with the nucleocapsid fraction of both BVs and ODVs, typically at one end of the nucleocapsids. In addition, the presence of sequence motifs with homology to invertebrate paramyosin proteins strongly supports a role for VP80 in the polar transport of nucleocapsids to the periphery of the nucleus on their way to the plasma membrane to form BVs and for assembly in the nuclear periphery to form ODVs for embedding in viral occlusion bodies.

Differential structure of the intronic promoter of the Bombyx mori A3 actin gene correlated with silkworm sensitivity/resistance to nucleopolyhedrovirus

Previous reports demonstrated that actin is necessary for nucleocapsid transport and viral gene expression during nucleopolyhedrovirus infection of Bombyx mori. The first intron of B. mori A3 actin contains a cryptic promoter that drives expression of a rare isoform. We detected differences in the size and nucleotide composition of the first intron of the A3 actin gene from B. mori strain C24A, which is more resistant to nucleopolyhedrovirus than the M11A strain (22 and 95% lethality, respectively). We sought to determine if resistance to BmMNPV infection and the A3 actin promoter structure are correlated. Intrinsically bent DNA sites in these sequences, which determine curved structures, were analyzed by electrophoretic mobility assays and the helical parameters ENDS ratio, roll and twist. We found both fragments to have non-centralized bent DNA sites with distinct ENDS ratio values, nucleotide positions and two-dimensional structures. Additionally, a conformational-sensitive gel electrophoresis assay identified an allelic variation found in strain M11A that is absent in strain C24A. These data suggest that A3 actin intronic sequence variations impair virus propagation and are markers of BmMNPV-resistant populations.

Actin-based motility drives baculovirus transit to the nucleus and cell surface

This virus takes a less-travelled cytoskeletal road both to reach its replication site in the nucleus and to get back to the plasma membrane to escape the host cell.

Most viruses move intracellularly to and from their sites of replication using microtubule-based mechanisms. In this study, we show that nucleocapsids of the baculovirus Autographa californica multiple nucleopolyhedrovirus undergo intracellular motility driven by actin polymerization. Motility requires the viral P78/83 capsid protein and the host Arp2/3 complex. Surprisingly, the virus directs two sequential and coordinated phases of actin-based motility. Immediately after cell entry, motility enables exploration of the cytoplasm and collision with the nuclear periphery, speeding nuclear entry and the initiation of viral gene expression. Nuclear entry itself requires transit through nuclear pore complexes. Later, after the onset of early gene expression, motility is required for accumulation of a subpopulation of nucleocapsids in the tips of actin-rich surface spikes. Temporal coordination of actin-based nuclear and surface translocation likely enables rapid transmission to neighboring cells during infection in insects and represents a distinctive evolutionary strategy for overcoming host defenses.

The putative pocket protein binding site of Autographa californica nucleopolyhedrovirus BV/ODV-C42 is required for virus-induced nuclear actin polymerization

Nuclear filamentous actin (F-actin) is essential for nucleocapsid morphogenesis of lepidopteran nucleopolyhedroviruses. Previously, we had demonstrated that Autographa californica multiple nucleopolyhedrovirus (AcMNPV) BV/ODV-C42 (C42) is involved in nuclear actin polymerization by recruiting P78/83, an AcMNPV orf9-encoded N-WASP homology protein that is capable of activating an actin-related-protein 2/3 (Arp2/3) complex to initiate actin polymerization, to the nucleus. To further investigate the role of C42 in virus-induced actin polymerization, the recombinant bacmid vAc(p78/83nls-gfp), with a c42 knockout, p78/83 tagged with a nuclear localization signal coding sequence, and egfp as a reporter gene under the control of the Pp10 promoter, was constructed and transfected to Sf9 cells. In the nuclei of vAc(p78/83nls-gfp)-transfected cells, polymerized F-actin filaments were absent, whereas other actin polymerization elements (i.e., P78/83, G-actin, and Arp2/3 complex) were present. This in vivo evidence indicated that C42 actively participates in the nuclear actin polymerization process as a key element, besides its role in recruiting P78/83 to the nucleus. In order to collect in vitro evidence for the participation of C42 in actin polymerization, an anti-C42 antibody was used to neutralize the viral nucleocapsid, which is capable of initiating actin polymerization in vitro. Both the kinetics of pyrene-actin polymerization and F-actin-specific staining by phalloidin indicated that anti-C42 can significantly attenuate the efficiency of F-actin formation compared to that with control antibodies. Furthermore, we have identified the putative pocket protein binding sequence (PPBS) on C42 that is essential for C42 to exert its function in nuclear actin polymerization.

Autographa californica multiple nucleopolyhedrovirus nucleocapsid protein BV/ODV-C42 mediates the nuclear entry of P78/83

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) BV/ODV-c42 (orf101; c42), which encodes a 41.5-kDa viral nucleocapsid protein with a putative nuclear localization signal (NLS) motif at the C terminus, is a highly conserved gene among members of the Baculoviridae family. C42 is demonstrated to be essential for AcMNPV propagation and can bind to nucleocapsid protein P78/83, a viral activator for the actin-related protein 2/3 (ARP2/3) complex to initiate nuclear actin polymerization, which is essential for viral nucleocapsid morphogenesis during AcMNPV infection. Here, we report the identification of a novel pathway through which c42 functions in nucleocapsid assembly. Cotransfection of Sf9 cells with c42 and p78/83 plasmids demonstrated that C42 was capable of recruiting P78/83 to the nuclei of uninfected cells and that the NLS motif of C42 was essential for this process. To validate this nuclear relocation mode in bacmid-transfected cells, a c42-disrupted bacmid (vAc(c42ko-gfp)) and rescued bacmids with wild-type c42 (vAc(c42res-gfp)) or with NLS coding sequence-mutated c42 (vAc(c42nls-gfp)) were prepared. By immuno-staining, P78/83 was found to be localized in the cytoplasm of either vAc(c42ko-gfp)- or vAc(c42nls-gfp)-transfected cells, whereas P78/83 was relocated to the nuclei of vAc(c42res-gfp)-transfected cells. Furthermore, F-actin-specific staining confirmed that there was no actin polymerization activity in the nuclei of either vAc(c42ko-gfp)- or vAc(c42nls-gfp)-transfected cells, which might be attributed to the absence of nuclear P78/83, an activator of the ARP2/3 complex to initiate nuclear actin polymerization. We therefore hypothesize a mode of action where C42 binds to P78/83 in the cytoplasm to form a protein complex and cotransports to the nucleus under the direction of the NLS motif in C42 during AcMNPV infection.

Identification of a hydrophobic domain of HA2 essential to morphogenesis of Helicoverpa armigera nucleopolyhedrovirus

The HA2 protein of the Helicoverpa armigera single-nucleocapsid nucleopolyhedrovirus (HearNPV) is a WASP homology protein capable of nucleating branched actin filaments in the presence of the Arp2/3 complex in vitro. To determine the role of ha2 in the HearNPV life cycle, ha2 knockout and ha2 repair bacmids were constructed. Transfection and infection analysis demonstrated that the ha2 null bacmid was unable to produce infectious budded virus (BV), while the repair bacmid rescued the defect. In vitro analysis demonstrated that the WCA domain of HA2 accelerates Arp2/3-mediated actin assembly and is indispensable to the function of HA2. However, analysis of the repaired recombinant with a series of truncated ha2 mutants demonstrated that the WCA domain was essential but not enough to yield infectious virions, and a hydrophobic domain (H domain) consisting of amino acids (aa) 167 to 193 played a pivotal role in the production of BV. Subcellular localization analysis with enhanced green fluorescent protein fusions showed that the H domain functioned as a nuclear localization signal. In addition, deletion of the C terminus of the ha2 product, a phosphatidylinositol 4-kinase homolog, dramatically decreased the viral titer, while deletion of 128 aa from the N terminus did not affect HA2 function.

Overexpression of cardiac actin with baculovirus is promoter dependent

The influence of the promoter and an N-terminal hexahistidine tag on human cardiac actin (ACTC) expression and function was investigated using four baculovirus constructs. It was found that both non-tagged ACTC and hisACTC expression from the p10 promoter was higher than from the polh promoter. Characterization showed that an N-terminal hexahistidine tag has a negative effect on ACTC. Recombinant ACTC inhibits DNase-I and binds myosin S1, indicative of proper folding. Our data support the hypothesis that the actin protein down-regulates the polh promoter.

Host filamentous actin is associated with Heliothis armigera single nucleopolyhedrosis virus (HaSNPV) nucleocapsid transport to the host nucleus

VP39 is the major capsid protein of Heliothis armigera nucleopolyhedrovirus (HaSNPV), and it might have induced the aggregation of host cellular actin in vitro in our previous study. We demonstrated here that VP39 could interact with host actin in vivo in Helicoverpazea (Hz-AM1 cells) through coimmunoprecipitation assay. With confocal immunofluorescence microscopy, it was confirmed further that the released HaSNPV nucleocapsids/VP39s in the host cytoplasm (0.5 hours after infection) colocalized where the actin aggregated and that the nucleocapsids/VP39s were transported from the host cytoplasm to the nucleus (2 hours after infection). Because cytochalasin D (CD) was used to prevent host global actin from forming filamentous structures, the infection efficiency of the recombinant virus HaSNPV/gfpdeltap74, with the gfp gene inserted into HaSNPV p74 gene loci, was decreased to 7.34%, whereas it was 34.7% in normal host cells and 55.7% in the cells whose microtubules had been destroyed by colchicin. Ultramicroscopy assay revealed that HaSNPV nucleocapsids could enter the cytoplasm of CD-treated cells but could not be transported to the nucleus, which resulted in the lower infection efficiency of HaSNPV/gfpdeltap74 in CD-treated cells. However, transportation of the nucleocapsids was not inhibited in colchicin-treated cells, demonstrating that the transportation of HaSNPV nucleocapsid from the cytoplasm to the nucleus was associated with actin filaments but not with microtubules, a conclusion that is also strongly supported by evidence from the RNAi interference of host actin during HaSNPV infection.

HA2 from the Helicoverpa armigera nucleopolyhedrovirus: a WASP-related protein that activates Arp2/3-induced actin filament formation

Filamentous actin is required for the productive replication of lepidopteran nucleopolyhedroviruses. We have demonstrated that nucleocapsids of the Helicoverpa armigera nucleopolyhedrovirus (HearNPV) are capable of nucleating actin polymerization in vitro in a dose-dependent manner. Actin polymerization is the main mechanism used in cell locomotion and is also utilized by the Listeria bacteria and by vaccinia virus for intracellular and intercellular movements. The WASP family of proteins has been shown to stimulate the assembly of branched actin filaments by the Arp2/3 complex. The process is conserved in eukaryotic cells. HearNPV ORF 2 (HA2), a WASP homologue, could nucleate branched actin filaments in the presence of Arp2/3 complex in vitro. We also demonstrate that HA2 co-localizes with Arp2/3 complex in the nucleus of infected cells, suggesting that HA2 and Arp2/3 complex are involved in nuclear actin polymerization. In summary, HA2 activates Arp2/3-induced actin filament network formation in vitro and in vivo.

The role of the cytoskeleton during viral infection

Upon infection, virions or subviral nucleoprotein complexes are transported from the cell surface to the site of viral transcription and replication. During viral egress, particles containing viral proteins and nucleic acids again move from the site of their synthesis to that of virus assembly and further to the plasma membrane. Because free diffusion of molecules larger than 500 kDa is restricted in the cytoplasm, viruses as well as cellular organelles employ active, energy-consuming enzymes for directed transport. This is particularly evident in the case of neurotropic viruses that travel long distances in the axon during retrograde or anterograde transport. Viruses use two strategies for intracellular transport: Viral components either hijack the cytoplasmic membrane traffic or they interact directly with the cytoskeletal transport machinery. In this review we describe how viruses--particularly members of the Herpesviridae, Adenoviridae, Parvoviridae, Poxviridae, and Baculoviridae--make use of the microtubule and the actin cytoskeleton. Analysing the underlying principles of viral cytosolic transport will be helpful in the design of viral vectors to be used in research as well as human gene therapy, and in the identification of new antiviral target molecules.

Identification of six Autographa californica multicapsid nucleopolyhedrovirus early genes that mediate nuclear localization of G-actin

Nuclear filamentous actin (F-actin) is required for nucleopolyhedrovirus (NPV) progeny production in NPV-infected, cultured lepidopteran cells. We have determined that monomeric G-actin is localized within the nuclei of host cells during the early stage of infection by Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV). With a library of cloned AcMNPV genomic fragments, along with a plasmid engineered to express enhanced green fluorescent protein-Bombyx mori G-actin in transient transfection experiments, we identified six AcMNPV early genes that mediate nuclear localization of G-actin in TN-368 cells: ie-1, pe38, he65, Ac004, Ac102, and Ac152. Within this subset, ie-1 and pe38 encode immediate-early transcriptional transactivators, he65 encodes a delayed-early product, and the products encoded by Ac004, Ac102, and Ac152 have not been characterized. We found that when driven by foreign promoters, ie-1, pe38, and Ac004 had to be expressed prior to Ac102 or he65 for nuclear G-actin to accumulate and that expression of Ac152 was no longer required. These results and others suggested that the product of Ac152 was a transactivator (directly or indirectly) of both Ac102 and he65 and that recruitment of G-actin to the nucleus was a temporally regulated process. Determining the functions of each of the six AcMNPV gene products with respect to our assay should provide valuable clues to basic cellular mechanisms of actin regulation and how AcMNPV infection affects them.

Interaction of Heliothis armigera nuclear polyhedrosis viral capsid protein with its host actin

In order to find the cellular interaction factors of the Heliothis armigera nuclear polyhedrosis virus capsid protein VP39, a Heliothis armigera cell cDNA library was constructed. Then VP39 was used as bait. The host actin gene was isolated from the cDNA library with the yeast two-hybrid system. This demonstrated that VP39 could interact with its host actin in yeast. In order to corroborate this interaction in vivo, the vp39 gene was fused with the green fluorescent protein gene in plasmid pEGFP39. The fusion protein was expressed in the Hz-AM1 cells under the control of the Autographa californica multiple nucleopolyhedrovirus immediate early gene promoter. The host actin was labeled specifically by the red fluorescence substance, tetramethy rhodamine isothicyanete-phalloidin. Observation under a fluorescence microscopy showed that VP39, which was indicated by green fluorescence, began to appear in the cells 6 h after being transfected with pEGFP39. Red actin cables were also formed in the cytoplasm at the same time. Actin was aggregated in the nucleus 9 h after the transfection. The green and red fluorescence always appeared in the same location of the cells, which demonstrated that VP39 could combine with the host actin. Such a combination would result in the actin skeleton rearrangement.

Actin rearrangement-inducing factor of baculoviruses is tyrosine phosphorylated and colocalizes to F-actin at the plasma membrane

In previous studies we have identified actin rearrangement-inducing factor 1 as an early gene product of Autographa californica multicapsid nuclear polyhedrosis virus that is involved in the remodeling of the actin cytoskeleton. We have constructed viral recombinants with a mutated Arif-1 open reading frame that confirm the causal link of Arif-1 expression and the actin rearrangement observed as accumulation of F-actin at the plasma membrane at 3 to 7 h postinfection. Infection with Arif mutant viruses leads to the loss of actin accumulation at the plasma membrane in TN-368 cells, although in the course of infection, early actin cables and nuclear F-actin are observed as in wild-type-infected cells. By immunofluorescence studies, we have demonstrated the localization of Arif-1 at the plasma membrane, and confocal imaging reveals the colocalization to F-actin. Accordingly, the approximately 47-kDa Arif-1 protein is observed exclusively in membrane fractions prepared at 4 to 48 h postinfection, with a decrease at 24 h postinfection. Phosphatase treatment suggests that Arif-1 is modified by phosphorylation. Antibodies against phosphotyrosine precipitate Arif-1 from membrane fractions, indicating that Arif-1 becomes tyrosine phosphorylated during the early and late phases of infection. In summary, our results indicate that functional Arif-1 is tyrosine phosphorylated and is located at the plasma membrane as a component of the actin rearrangement-inducing complex.

Filamentous actin is required for lepidopteran nucleopolyhedrovirus progeny production

Autographa californica M nucleopolyhedrovirus (AcMNPV) is the prototypical member of the NUCLEOPOLYHEDROSIS: genus of the BACULOVIRIDAE:, a family of large, double-stranded DNA viruses that are highly diverse. Nucleocapsid morphogenesis of AcMNPV and others in the NUCLEOPOLYHEDROVIRUS: genus takes place within the nuclei of infected host cells. Previously, we showed that filamentous actin (F-actin) is essential for this process to occur in AcMNPV-infected cells, an unprecedented finding for a DNA virus that replicates within the nucleus. Because of the fundamental importance of this requirement to our understanding of virus-host interactions, and because of the diversity of viruses included within the Nucleopolyhedrovirus genus, we were compelled to determine whether the replication of other nucleopolyhedroviruses was also F-actin dependent. We report here that progeny virus production of six other lepidopteran nucleopolyhedroviruses, representing both phylogenetic groups I and II within the genus, is also F-actin dependent. The six viruses studied (Spodoptera frugiperda MNPV, Bombyx mori NPV, Orgyia pseudotsugata MNPV, Lymantria dispar MNPV, Anticarsia gemmatalis MNPV and Helicoverpa zea SNPV) were unable to produce progeny in the presence of either cytochalasin D or latrunculin A, two actin-binding agents that interfere with F-actin-dependent processes but differ in their modes of action. F-actin-dependent progeny morphogenesis, therefore, appears to be a characteristic common among viruses in this genus that have lepidopteran hosts.