Baculoviridae
Baculoviridae is a family of viruses. Arthropods, lepidoptera, hymenoptera, diptera, and decapoda serve as natural hosts. There are currently 84 species in this family, divided among four genera.
Baculoviruses are known to infect invertebrates, with over 600 host species having been described. Immature forms of moth species are the most common hosts, but these viruses have also been found infecting sawflies, mosquitoes, and shrimp. Although baculoviruses are capable of entering mammalian cells in culture
they are not known to be capable of replication in mammalian or other vertebrate animal cells.
Starting in the 1940s they were used and studied widely as biopesticides in crop fields. Baculoviruses contain circular double-stranded genome ranging from 80 to 180 kbp.
Historical influence
The earliest records of baculoviruses can be found in the literature from as early as the sixteenth century in reports of "wilting disease" infecting silkworm larvae. Starting in the 1940s the viruses were used and studied widely as biopesticides in crop fields. Since the 1990s they have been employed to produce complex eukaryotic proteins in insect cell cultures. These recombinant proteins have been used in research and as vaccines in both human and veterinary medical treatments. More recently it has been found that baculoviruses can transduce mammalian cells with a suitable promoter.Baculovirus life cycle
The baculovirus life cycle involves two distinct forms of virus. Occlusion derived virus is present in a protein matrix and is responsible for the primary infection of the host while the budded virus is released from the infected host cells later during the secondary infection.Baculoviruses have very species-specific tropisms among the invertebrates with over 700 host species having been described. Immature forms of moth species are the most common hosts, but these viruses have also been found infecting sawflies, mosquitoes, and shrimp.
Typically, the initial infection occurs when a susceptible host insect feeds on plants that are contaminated with the occluded form of the virus. The protein matrix dissolves in the alkaline environment of the host midgut, releasing ODV that then fuse to the columnar epithelial cell membrane of the host intestine and are taken into the cell in endosomes. Nucleocapsids escape from the endosomes and are transported to nucleus. This step is possibly mediated by actin filaments. Viral transcription and replication occur in the cell nucleus and new BV particles are budded out from the basolateral side to spread the infection systemically. During budding, BV acquires a loosely fitting host cell membrane with expressed and displayed viral glycoproteins.
Baculovirus infection can be divided to three distinct phases:
- Early,
- Late
- Very late phase
When infecting a caterpillar, the advanced stages of infection cause the host to feed without resting, and then to climb to the higher parts of trees, including exposed places they would normally avoid due to the risk of predators. This is an advantage for the virus if it can drip down onto leaves which will be consumed by new hosts.
Structure of the virion
The most studied baculovirus is Autographa californica multicapsid nucleopolyhedrovirus. The virus was originally isolated from the alfalfa looper and contains a 134-kbp genome with 154 open reading frames. The major capsid protein VP39 together with some minor proteins forms the nucleocapsid that encloses the DNA with p6.9 protein.BV acquires its envelope from the cell membrane and requires a glycoprotein, gp64, to be able to spread systemic infection. This protein forms structures called peplomers on one end of the budded virus particle but is not found on ODV. Some differences also exist in the lipid composition of the viral envelope of the two forms. While the BV envelope consists of phosphatidylserine, ODV contains phosphatidylcholine and phosphatidylethanolamine.
A nucleocapsid assembly-essential element was identified in the AcMNPV genome. The NAE is an internal cis-element within the ac83 gene. The nucleocapsid assembly is not dependent on the Ac83 protein product.
Major envelope glycoprotein gp64
During periods of evolution, the baculoviral envelope glycoproteins have undergone changes. Ld130, also known as baculovirus F-protein from Lymantria dispar is suggested to be an ancestral envelope fusion protein which has been replaced by non-orthologous gene replacement with gp64 in AcMNPV, Bombyx mori and Orgyia pseudotsugata while they still retain the ld130 gene.Gp64 is a homotrimeric membrane glycoprotein which is polarly present on the rod-shaped virion. It consists of 512 amino acids with four glycosylation sites at asparagine residues and has a N-terminal signal sequence, oligomerization and fusion domain and a hydrophobic transmembrane domain near the C-terminus.
It is produced in both early and late phases of the infection cycle with a maximal rate of synthesis occurring in 24–26 h p.i.. Trimerization with intermolecular cysteine-bonds seems to be a crucial step for protein transport to cell surface, since only 33% of synthesized protein reaches cell surface as monomeric gp64 is degraded within the cells.
Gp64 is essential for efficient budding of the virion and for the cell-to-cell transmission during the infection cycle as well as viral entry i.e. causing viral trophism and endosome mediated uptake to the cell. The major function of the gp64 envelope protein is to cause the pH-mediated envelope fusion to the endosome. Although gp64 has variety of essential functions, it has been reported that gp64-null baculoviruses can be substituted with other viral glycoproteins such as Ld130, G-protein of Vesicular stomatitis virus. These substitutions will result in functional virons.
Applications
Baculovirus expression in insect cells represents a robust method for producing recombinant glycoproteins or membrane proteins. Baculovirus-produced proteins are currently under study as therapeutic cancer vaccines with several immunologic advantages over proteins derived from mammalian sources.Biosafety
Baculoviruses are incapable of replicating within the cells of mammals and plants. They have a restricted range of hosts that they can infect that is typically restricted to a limited number of closely related insect species. Because baculoviruses are not harmful to humans, they are considered a safe option for use in research applications. They are also used as biological agents as in the case of the Indian mealmoth, a grain-feeding pest.Taxonomy
The name of this family has been derived from the latin word baculus, meaning stick. The family has been divided into four genera: Alphabaculovirus, Betabaculovirus, Gammabaculovirus and Deltabaculovirus.Group: dsDNA