Cytoplasmic polyhedrosis virus or CPV of the genus
Cypovirus of
Reoviridae family [
1,
2] infects the midgut of the wide range of insects belonging to the order Diptera, Hymenoptera and Lepidoptera [
3,
4]. Like other members of
Reoviridae, CPV genome is also composed of 10 double stranded RNA segments (dsRNA) (S1-S10) [
2]. A small eleventh segment (S11) has been reported in some cases such as
Bombyx mori CPV (BmCPV) [
5] and
Trychoplusia ni CPV (TnCPV) [
6]. Each dsRNA segment is composed of a plus mRNA strand and it's complementary minus strand in an end to end base pair configuration except for a protruding 5' cap on the plus strand. On the basis of electrophoretic migration patterns of the dsRNA segments in agarose or acrylamide gels, CPVs have been classified into 16 different types [
1,
7]. CPVs are self competent for transcription, possessing all the enzymes necessary for mRNA synthesis and processing [
8]. BmCPV, the type Cypovirus, has a single layer capsid made up of 120 copies of the major capsid protein, VP1, which is decorated with 12 turrets on its icosahedral vertices [
9,
10]. These hollow turrets are involved in post-transcriptional processing of viral mRNA and provide a channel through which newly synthesized 5'capped viral RNA are released from the capsid into the cytoplasm of infected cells [
10,
11]. After translation of this mRNA into capsid, polymerase and other proteins, they assembled into viral procapsid within which one copy of each genome segments plus polarity RNA are packaged and replicated to form dsRNA. CPV capsids thus formed can be released as non-occluded virus particles to directly infect fresh neighboring cells or occluded in a viral protein matrix called polyhedrin to form polyhedra [
12]. It has been reported that VP1 protein, encoded by genome segment 1 of BmCPV, can self assemble to form single shelled virus like particles (VLPs) [
13,
14] and their stability is maintained by interaction with VP3 and VP4 proteins encoded by genome segments 3 and 4, respectively [
15,
16]. Recent cryo-electron microscopic study has shown the region of capsid protein directly interacting with viral RNA indicating the role of capsid in RNA packaging, replication and transcription [
17]. Therefore, understanding the assembly of capsid not only provides insight into in the virus life cycle but also helps to develop mechanism for the disruption of virus assembly for therapeutic application [
18]. But besides BmCPV, capsids of other CPVs have not been studied well although all the genome segments of DpCPV, LdCPV and TnCPV have been cloned and sequenced [
6,
19‐
21].
Antheraea mylitta cytoplasmic polyhedrosis virus (AmCPV) is one of the most widespread pathogens of Indian non-mulberry silkworm,
A. mylitta. CPV-infected
A. myllita larvae develop chronic diarrhea that eventually leads to a condition known as "Grasserie" and ultimate death [
22]. Almost 20-30% larval mortality occurs annually due to this virus attack [
22]. We have previously characterized the structure of AmCPV by electron microscopy and its genome by electrophoresis which reveals that it is similar to that of a type- 4 CPV and consists of 11 ds RNA molecules [
23]. We have also reported that the genome segments 6, 7, 8 of AmCPV encode viral structural proteins [
24‐
26], segment 2 encodes viral RNA dependent RNA polymerase [
27], segment 9 encodes a nonstructural protein, NSP38, having RNA binding property [
28], segment 10 codes for polyhedrin [
29] and segment 11 does not code for any protein [
26]. But the genome segments encoding viral capsid proteins have not been characterized. Here, we report molecular cloning, sequencing and expression of S1 and S3 of AmCPV in
E. Coli via bacterial expression vector as well as in insect cells using a baculovirus system and show by functional analysis that S3 encoded protein can self assemble into capsid and S1 encoded protein remains associated with the capsid to maintain its stability.