Functional characterization of hesp018, a baculovirus-encoded serpin gene

Abstract

The serpin family of serine proteinase inhibitors plays key roles in a variety of biochemical pathways. In insects, one of the important functions carried out by serpins is regulation of the phenoloxidase (PO) cascade -a pathway that produces melanin and other compounds that are important in insect humoral immunity. Recent sequencing of the baculovirus Hemileuca sp. nucleopolyhedrovirus (HespNPV) genome revealed the presence of a gene, hesp018, with homology to insect serpins. To our knowledge, hesp018 is the first viral serpin homologue to be characterized outside of the chordopoxviruses. The Hesp018 protein was found to be a functional serpin with inhibitory activity against a subset of serine proteinases. Hesp018 also inhibited PO activation when mixed with lepidopteran haemolymph. The Hesp018 protein was secreted when expressed in lepidopteran cells and a baculovirus expressing Hesp018 exhibited accelerated production of viral progeny during in vitro infection. Expression of Hesp018 also reduced caspase activity induced by baculovirus infection, but caused increased cathepsin activity. In infected insect larvae, expression of Hesp018 resulted in faster larval melanization, consistent with increased activity of viral cathepsin. Finally, expression of Hesp018 increased the virulence of a prototype baculovirus by fourfold in orally infected neonate Trichoplusia ni larvae. Based on our observations, we hypothesize that hesp018 may have been retained in HespNPV due to its ability to inhibit the activity of select host proteinases, possibly including proteinases involved in the PO response, during infection of host insects.

Inhibition of melanization by serpin-5 and serpin-9 promotes baculovirus infection in cotton bollworm Helicoverpa armigera

“…As the increased expression of components of the melanization cascade in both iserpin-5/NPV and iserpin-9/NPV insects were observed (S6B Fig). Interestingly, a baculovirus encoded serpin has been characterized as an inhibitor of host melanization and its expression increased the virulence of the virus by four-fold in Trichoplusia ni larvae [42]. However, this molecule is not an ortholog of serpin-5 or serpin-9 in H .…”

Covert Infection of Insects by Baculoviruses

“…The Hesp018 protein was shown to inhibit prophenoloxidase activation in hemolymph, suggesting that it may contribute to supressing the host humoral immune response. Expression of the serpin in a recombinant nucleopolyhedrovirus also resulted in accelerated production of BVs in cell culture and a fourfold reduction in the quantity of OBs required for lethal infection of Trichoplusia ni larvae, indicating a role in modulating virulence (Ardisson-Araujo et al, 2015). …”

Reaching the melting point: Degradative enzymes and protease inhibitors involved in baculovirus infection and dissemination

“…Two types of protease inhibitors are found in both baculoviruses and poxviruses: 1) the P35/P49 family of caspase inhibitors, which are found in some baculoviruses and entomopoxviruses (Means et al, 2007), and 2) serpins, which are encoded by most members of the vertebrate-infecting chordopox viruses and in a single known baculovirus (Ardisson-Araujo et al, 2014; Rohrmann et al, 2013). It should be noted that while most baculoviruses encode homologs of the inhibitor of apoptosis (IAP) family of proteins, and some cellular IAPs can directly bind and inhibit caspases, the baculoviral IAPs do not appear to interact directly with caspases (Clem, 2007), and so are not included as protease inhibitors.…”