Functional characterization of Autographa californica multiple nucleopolyhedrovirus ORF43 and phenotypic changes of ORF43-knockout mutant

Successful Rescue of Synthetic AcMNPV with a ~17 kb Deletion in the C1 Region of the Genome

Baculoviruses have been widely used as expression vectors. However, numerous genes in the baculoviral genome are non-essential for cellular infection and protein expression, making the optimisation of baculovirus expression vectors possible. We used a synthetic biological method to reduce the number of genes in a partial region of the autograph californica multiple nucleopolyhedrovirus (AcMNPV), the most widely used baculovirus expression vector. The C1 region of the AcMNPV is 46.4 kb and is subdivided into B1, B2, and B3 fragments. We first designed modified B1, B2, and B3 fragments by deleting the non-essential genes, and then synthesised complete viral genomes containing either individual modified B fragments or joint modified B fragments through transformation-related recombination in yeast. The synthetic genomes were then transfected into Sf9 cells to rescue the progeny viruses and test their infectivity. The design-build-test cycle was repeated until the ultimately rescued virus could produce progeny viruses efficiently. Finally, AcMNPV-Syn-mC1-1.1 by deleting approximately 17.2 kb, including 20 ORFs, in the C1 region, was obtained. This is essential to the synthesis of a minimal AcMNPV genome that can generate infectious progeny viruses and can be further used to optimise the foundation of baculovirus expression vectors.

Systematic analysis of nuclear localization of Autographa californica multiple nucleopolyhedrovirus proteins

Baculoviruses are large DNA viruses that replicate within the nucleus of infected host cells. Therefore, many viral proteins must gain access to the nucleus for efficient viral genome replication, gene transcription and virion assembly. To date, the global protein localization pattern of baculoviral proteins is unknown. In this study, we systematically analysed the nuclear localization of 154 ORFs encoded by the prototypic baculovirus, Autographa californica multiple nucleopolyhedrovirus (AcMNPV), either during transient expression or with super-infection of the virus. By transient expression of vectors containing egfp-fused ORFs, we found that in the absence of virus infection, 25 viral proteins were localized in the nucleus. Most of these, which we called ‘auto-nuclear localization’ proteins, are related to virus replication, transcription or virion structure, and 20 of them contain predicted classical nuclear localization signal. Upon virus infection, 11 proteins, which originally localized in the cytoplasm or both cytoplasm and nucleus in the transfection assays, were completely translocated into the nucleus, suggesting that their nuclear import is facilitated by other viral or host proteins. Further co-transfection experiments identified that four of the 11 proteins, including P143, P33, AC73 and AC114, were imported into the nucleus with the assistance of the auto-nuclear localization proteins LEF-3 (for P143), TLP (for P33) and VP80 (for both AC73 and AC114). This study presents the first global nuclear localization profile of AcMNPV proteins and provides useful information for further elucidation of the mechanisms of baculovirus nuclear entry and gene functions.

The autographa californica multiple nucleopolyhedrovirus Ac12: A non-essential F box-like protein that interacts with cellular SKP1 component of the E3 ubiquitin ligase complex

The Autographa californica multiple nucleopolyhedrovirus (AcMNPV) ac12 gene, which is conserved in ten other baculovirus, codes a predicted 217 amino acid protein of unknown function. In this study, we investigated the role of ac12 during baculovirus infection, by generating an ac12 knockout virus. The transfection of the recombinant genome in insect cells resulted in unaltered viral dispersion and occlusion body production when compared to the control bacmid. This finding demonstrates that ac12 is a non-essential gene. Transmission and scanning electron microscopy (SEM) analyses showed that ac12 knockout virus produced occlusion bodies morphologically similar to those obtained with the control and capable to occlude virions. However, a slight but significant size difference was detected by SEM observation of purified occlusion bodies. This difference suggests that ac12 may be involved in regulatory pathways of polyhedrin production or occlusion body assembly without affecting either viral occlusion or oral infectivity in Rachiplusia nu larvae. This was evidenced by bioassays that showed no significant differences in the conditions tested. A qPCR analysis of viral gene expression during infection evidenced regulatory effects of ac12 over some representative genes of different stages of the viral cycle. In this study, we also showed that ac12 is transcribed at early times after infection and remains detectable up to 72 hours post-infection. The mRNA is translated during the infection and results in a protein that encodes an F-box domain that interacts in vivo and in vitro with S phase kinase associated protein 1 (SKP1) adaptor protein, which is potentially involved in protein ubiquitination pathways.

Protein composition analysis of polyhedra matrix of Bombyx mori nucleopolyhedrovirus (BmNPV) showed powerful capacity of polyhedra to encapsulate foreign proteins

Polyhedra can encapsulate other proteins and have potential applications as protein stabilizers. The extremely stable polyhedra matrix may provide a platform for future engineered micro-crystal devices. However, the protein composition of the polyhedra matrix remains largely unknown. In this study, the occlusion-derived virus (ODV)-removed BmNPV polyhedra matrix fraction was subjected to SDS-PAGE and then an LC-ESI-MS/MS analysis using a Thermo Scientific Q Exactive mass spectrometer. In total, 28 host and 91 viral proteins were identified. The host components were grouped into one of six categories, i.e., chaperones, ubiquitin and related proteins, host helicases, cytoskeleton-related proteins, RNA-binding proteins and others, according to their predicted Pfam domain(s). Most viral proteins may not be essential for polyhedra assembly, as evidenced by studies in the literature showing that polyhedra formation occurs in the nucleus upon the disruption of individual genes. The structural role of these proteins in baculovirus replication will be of significant interest in future studies. The immobilization of enhanced green fluorescent protein (eGFP) into the polyhedra by fusing with the C-terminus of BM134 that is encoded by open reading frame (ORF) 134 suggested that the polyhedra had a powerful capacity to trap foreign proteins, and BM134 was a potential carrier for incorporating proteins of interest into the polyhedra.

Baculovirus nucleocapsid aggregation (MNPV vs SNPV): an evolutionary strategy, or a product of replication conditions?

Lepidopteran nucleopolyhedroviruses are members of the Baculoviridae and have been categorized as having two morphotypes of occluded virions: multiple nucleocapsids or single nucleocapsids within the virion envelope. Although it is a definitive characteristic of specific viruses, it appears to lack a defined genetic basis and is independent of virus phylogeny. This review summarizes the factors that appear to influence this trait and the role that it may play in virus biology.

Disruption of the baculovirus core gene ac78 results in decreased production of multiple nucleocapsid-enveloped occlusion-derived virions and the failure of primary infection in vivo

The Autographa californica multiple nucleopolyhedrovirus (AcMNPV) ac78 gene is one of the baculovirus core genes. Recent studies showed that ac78 is essential for budded virion (BV) production and the embedding of occlusion-derived virion (ODV) into occlusion body during the AcMNPV life cycle. Here, we report that an ac78-knockout AcMNPV (vAc78KO) constructed in this study had different phenotypes than those described in the previous studies. A few infectious BVs were detected using titer assays, immunoblot analyses and plaque assays, indicating that ac78 is not essential for BV formation. Electron microscopy confirmed that the ac78 deletion did not affect nucleocapsid assembly and ODV formation. However, the numbers of multiple nucleocapsid-enveloped ODVs and ODV-embedded occlusion bodies were significantly decreased. Subsequently, the highly conserved amino acid residues 2-25 and 64-88 of Ac78, which are homologous to an oxidoreductase and cytochrome c oxidase, respectively, were demonstrated to play a crucial role in the morphogenesis of multiple nucleocapsid-enveloped ODV. Immunoblot analysis found that Ac78 was an ODV envelope-associated protein. Consistently, amino acid residues 56-93 of Ac78 were identified as an inner nuclear membrane sorting motif, which may direct the localization of Ac78 to the ODV envelope. In vivo infectivity assays showed that the occlusion bodies of vAc78KO were unable to establish primary infection in the midgut of Trichoplusia ni larvae. Taken together, our results suggest that ac78 plays an important role in BV production and proper multiple nucleocapsid-enveloped ODV formation, as well as AcMNPV primary infection in vivo.

The Trichoplusia ni single nucleopolyhedrovirus tn79 gene encodes a functional sulfhydryl oxidase enzyme that is able to support the replication of Autographa californica multiple nucleopolyhedrovirus lacking the sulfhydryl oxidase ac92 gene

The Autographa californica multiple nucleopolyhedrovirus ac92 is a conserved baculovirus gene with homology to flavin adenine dinucleotide-linked sulfhydryl oxidases. Its product, Ac92, is a functional sulfhydryl oxidase. Deletion of ac92 results in almost negligible levels of budded virus (BV) production, defects in occlusion-derived virus (ODV) co-envelopment and their inefficient incorporation into occlusion bodies. To determine the role of sulfhydryl oxidation in the production of BV, envelopment of nucleocapsids, and nucleocapsid incorporation into occlusion bodies, the Trichoplusia ni single nucleopolyhedrovirus ortholog, tn79, was substituted for ac92. Tn79 was found to be an active sulfhydryl oxidase that substituted for Ac92, resulting in the production of infectious BV, albeit about 10-fold less than an ac92-containing virus. Tn79 rescued defects in ODV morphogenesis caused by a lack of ac92. Active Tn79 sulfhydryl oxidase activity is required for efficient BV production, ODV envelopment, and their subsequent incorporation into occlusion bodies in the absence of ac92.