Characterization of JKT-7400, an orbivirus which grows in Aedes albopictus mosquito cells: evidence pointing to a minor virion protein, VP6, as the RNA guanylyltransferase

Genome segment 4 of Antheraea mylitta cytoplasmic polyhedrosis virus encodes RNA triphosphatase and methyltransferases

Cloning and sequencing of Antheraea mylitta cytoplasmic polyhedrosis virus (AmCPV) genome segment S4 showed that it consists of 3410 nt with a single ORF of 1110 aa which could encode a protein of ~127 kDa (p127). Bioinformatics analysis showed the presence of a 5' RNA triphosphatase (RTPase) domain (LRDR), a S-adenosyl-l-methionine (SAM)-binding (GxGxG) motif and the KDKE tetrad of 2'-O-methyltransferase (MTase), which suggested that S4 may encode RTPase and MTase. The ORF of S4 was expressed in Escherichia coli as a His-tagged fusion protein and purified by nickel-nitrilotriacetic acid affinity chromatography. Biochemical analysis of recombinant p127 showed its RTPase as well as SAM-dependent guanine N(7)-and ribose 2'-O-MTase activities. A MTase assay using in vitro transcribed AmCPV S2 RNA having a 5' G*pppG end showed that guanine N(7) methylation occurred prior to the ribose 2'-O methylation to yield a m(7)GpppG/m(7)GpppGm RNA cap. Mutagenesis of the SAM-binding (GxGxG) motif (G831A) completely abolished N(7)- and 2'-O-MTase activities, indicating the importance of these residues for capping. From the kinetic analysis, the Km values of N(7)-MTase for SAM and RNA were calculated as 4.41 and 0.39 µM, respectively. These results suggested that AmCPV S4-encoded p127 catalyses RTPase and two cap methylation reactions for capping the 5' end of viral RNA.

Genome segment 5 of Antheraea mylitta cytoplasmic polyhedrosis virus encodes a bona fide guanylyltransferase

BACKGROUND

Antheraea mylitta cytoplasmic polyhedrosis virus (AmCPV), a cypovirus of Reoviridae family, infects non mulberry Indian silk worm, Antheraea mylitta, and contains eleven segmented double stranded RNA in its genome (S1-S11). Some of its genome segments (S1-S3, and S6-S11) have been previously characterized but genome segment encoding the viral guanylyltransferase which helps in RNA capping has not been characterized.

RESULTS

In this study genome segment 5 (S5) of AmCPV was converted to cDNA, cloned and sequenced. S5 consisted of 2180 nucleotides, with one long ORF of 1818 nucleotides and could encode a protein of 606 amino acids with molecular mass of ~65 kDa (p65). Bioinformatics analysis showed presence of KLRS and HxnH motifs as observed in some other reoviral guanylyltransferase and suggests that S5 may encodes viral guanylyltransferase. The ORF of S5 was expressed in E. coli as 65 kDa his tagged fusion protein, purified through Ni-NTA chromatography and polyclonal antibody was raised. Immunoblot analysis of virion particles with the purified antibody showed specific immunoreactive band and suggests p65 as a viral structural protein. Functional analysis showed that recombinant p65 possesses guanylyltransferase activity, and transfers GMP moiety to the 5' diphosphate (A/G) ended viral RNA after the formation of p65-GMP complex for capping. Kinetic analysis showed K(m) of this enzyme for GTP and RNA was 34.24 uM and 98.35 nM, respectively. Site directed mutagenesis at K21A in KLRS motif, and H93A or H105A in HxnH motif completely abolished the autoguanylylation activity and indicates importance of these residues at these sites. Thermodynamic analysis showed p65-GTP interaction was primarily driven by enthalpy (ΔH = -399.1 ± 4.1 kJ/mol) whereas the p65-RNA interaction by favorable entropy (0.043 ± 0.0049 kJ/ mol).

CONCLUSION

Viral capping enzymes play a critical role in the post transcriptional or post replication modification in case of RNA virus. Our results of cloning, sequencing and functional analysis of AmCPV S5 indicates that S5 encoded p65 through its guanylyltransferase activity can transfer guanine residue to the 5' end of viral RNA for capping. Further studies will help to understand complete capping process of cypoviral RNA during viral replication within the viral capsid.

Capping and methylation of mRNA by purified recombinant VP4 protein of bluetongue virus

The core of bluetongue virus (BTV) is a multienzyme complex composed of two major proteins (VP7 and VP3) and three minor proteins (VP1, VP4, and VP6) in addition to the viral genome. The core is transcriptionally active and produces capped mRNA from which all BTV proteins are translated, but the relative role of each core component in the overall reaction process remains unclear. Previously we showed that the 76-kDa VP4 protein possesses guanylyltransferase activity, a necessary part of the RNA capping reaction. Here, through the use of highly purified (>95%) VP4 and synthetic core-like particles containing VP4, we have investigated the extent to which this protein is also responsible for other activities associated with cap formation. We show that VP4 catalyzes the conversion of unmethylated GpppG or in vitro-produced uncapped BTV RNA transcripts to m7GpppGm in the presence of S-adenosyl-L-methionine. Analysis of the methylated products of the reaction by HPLC identified both methyltransferase type 1 and type 2 activities associated with VP4, demonstrating that the complete BTV capping reaction is associated with this one protein.

Binding site for S-adenosyl-L-methionine in a central region of mammalian reovirus lambda2 protein. Evidence for activities in mRNA cap methylation

One or more proteins in mammalian reovirus core particles mediate two RNA methylation activities, (guanosine-7-N)-methyltransferase and (guanosine-2'-O)-methyltransferase, that contribute to forming the 5' cap 1 structure on viral mRNA. We used UV irradiation to identify core proteins that bind S-adenosyl-L-methionine (SAM), the methyl-group donor for both methyltransferases. A [methyl-3H]SAM-binding site was observed among the reovirus lambda proteins; was shown to be specific by competition with low levels of S-adenosyl-L-homocysteine, the product of methyl-group transfer from SAM; and was subsequently localized to protein lambda2. lambda2 mediates the guanylyltransferase reaction in cap formation and was previously proposed to mediate one or both methylation reactions as well. SAM binding was demonstrated for both lambda2 in cores and lambda2 expressed in insect cells from a recombinant baculovirus. Using three different methods to cleave lambda2, a binding site for SAM was tentatively localized to a central region of lambda2, between residues 792 and 1100, which includes a smaller region with sequence similarity to the SAM-binding pocket of other methyltransferases. Alanine substitutions at positions 827 and 829 within this predicted binding region greatly reduced the capacity of baculovirus-expressed lambda2 protein to undergo UV cross-linking to SAM but had no effects on either the guanylyltransferase activity of this protein or its conformation as judged by partial proteolysis, suggesting that one or both of these residues is essential for SAM binding. Based on these findings, we propose that the two methyltransferase activities involved in mRNA capping by reovirus cores utilize a single SAM-binding pocket within a central region of lambda2.

Methyltransferase activity of the insect orbivirus JKT-7400: photoaffinity labeling of a minor virion protein, VP4, with S-adenosylmethionine

JKT-7400 virus is an orbivirus originally isolated from Culex mosquitoes. In earlier work we had described the viral structural proteins and presented evidence suggesting that a minor protein, VP6, located in the viral core was the viral guanylyltransferase. We now show that gradient-purified JKT-7400 virions possess a methyltransferase (MTase) activity which can use GTP or GDP as the methyl acceptor. The apparent Km of the MTase for S-adenosylmethionine (AdoMet) was 25 microM. Photoaffinity labeling experiments in which 3H-[methyl]-AdoMet was incubated with virions or viral cores demonstrated labeling of VP4, a minor protein present in the viral core, suggesting that this protein is the viral MTase. Labeling of VP4 was inhibited by addition of unlabeled AdoMet or S-adenosylhomocysteine (AdoHcy).