Abstract

Baculoviruses are arthropod-specific large DNA viruses that orally infect the larvae of lepidopteran, hymenopteran and dipteran insect species. These larvae become infected when they eat a food source that is contaminated with viral occlusion bodies (OBs). These OBs contain occlusion-derived viruses (ODVs), which are released upon ingestion of the OBs and infect the endothelial midgut cells. At least nine different ODV envelope proteins are essential for this oral infectivity and these are denoted per os infectivity factors (PIFs). Seven of these PIFs form a complex, consisting of PIF1, 2, 3 and 4 that form a stable core complex and PIF0 (P74), PIF6 and PIF8 (P95) that associate with this complex with lower affinity than the core components. The existence of a PIF complex and the fact that the pif genes are conserved in baculovirus genomes suggests that PIF-proteins cooperatively mediate oral infectivity rather than as individual functional entities. This review therefore discusses the knowledge obtained for individual PIFs in light of their relationship with other members of the PIF complex.

Highlights

  • The Baculoviridae form a family of arthropod-specific large double stranded DNA viruses that infect insect larvae

  • Viruses classified in the genera Alphabaculovirus and Betabaculoviruses infect the larvae of lepidopteran insect species

  • occlusion-derived viruses (ODVs) bind to the microvilli of midgut columnar cells and the viral envelope fuses with the cell membrane, releasing nucleocapsids into the cell. This process of virus entry is mediated by a specific set of proteins in the ODV-envelope, the so-called per os infectivity factors (PIFs)

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Summary

Introduction

The Baculoviridae form a family of arthropod-specific large double stranded DNA viruses that infect insect larvae. Each BV consists of a single nucleocapsid surrounded by an envelope derived from the plasma membrane of a host cell These BVs display a fusion protein to allow cell entry (either GP64 or F protein). ODVs bind to the microvilli of midgut columnar cells and the viral envelope fuses with the cell membrane, releasing nucleocapsids into the cell This process of virus entry is mediated by a specific set of proteins in the ODV-envelope, the so-called per os infectivity factors (PIFs). For Autographa californica multiple nucleopolyhedrovirus (AcMNPV), Peng and co-workers showed that PIFs form a large protein complex, suggesting that these proteins mediate midgut entry of baculovirus ODVs in a cooperative manner [4,5]. We summarize current knowledge of PIF-protein structure and function

Definition and General Features of PIFs
The AcMNPV
Model ofthe theaffinity
PIF1 and 2 Mediate Binding to the Columnar Cell Microvilli
PIF3 Mediate Oral Infectivity after Binding and Fusion
The Role of PIF1-3 in Host-Range Determination
PIF4 Provides Proteolytic Resistance to the Stable Core
PIF0 Mediates ODV Binding with the Midgut Epithelium
PIF0 Is Cleaved by Co-Occluded Proteases and Trypsins
PIF6 Is Also a Component of the ODV Entry Complex
The Zinc-Finger Domain of PIF8 Is Important for Oral Infectivity
PIF5 Might Operate Independent from the ODV Entry Complex
ODV-E66 Might Retrospectively Be Assigned as PIF9
Conclusions and and Perspectives

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