DNA-independent ATPase activity of the Trichoplusia ni granulovirus DNA helicase

D. K. Bideshi1 and B. A. Federici1

Graduate Program in Genetics1 and Department of Entomology2, University of California, Riverside, CA 92521, USA

Author for correspondence: Brian Federici (at Department of Entomology). Fax +1 909 787 3086 or 3681. e-mail brian.federici{at}ucr.edu


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DNA helicases of baculoviruses are essential for virus replication and have been implicated as molecular determinants of host range. Although these proteins contain seven motifs (I, Ia, II–VI) characteristic of DNA helicases, the two most important characteristics of helicases – duplex-DNA unwinding and ATPase activity – have not been demonstrated. In the present study, a recombinant putative DNA helicase (rP137) of Trichoplusia ni granulovirus (TnGV) was purified from insect cells infected with a recombinant Autographa californica multicapsid nucleopolyhedrovirus that overproduced rP137. The rP137 protein exhibited an intrinsic DNA-independent ATPase activity that required Mg2+ as a co-factor, an activity that was reduced in the presence of TnGV and phage {lambda} DNAs. These results provide further evidence that baculovirus helicase genes encode proteins with biochemical properties similar to those of classical DNA helicases.


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The family Baculoviridae is composed of a large and diverse group of circular dsDNA occluded viruses isolated primarily from lepidopteran insects (Martignoni & Iwai, 1986 ; Volkman et al., 1995 ). Two groups are recognized based on occlusion body morphology: the nucleopolyhedroviruses (NPVs) and granuloviruses (GVs). Though the NPVs and GVs are related phylogenetically (Zanotto et al., 1993 ; Cowan et al., 1994 ; Bulach et al., 1999 ), members within each group vary in their host range, tissue tropism and pathobiology (Federici, 1997 ). The genetic determinants and molecular mechanisms responsible for these differences are largely unknown.

In this regard, studies on baculovirus putative DNA helicases are of considerable interest because these enzymes are essential for virus replication and potentially play a significant role in baculovirus host range. For example, in the accompanying paper (Bideshi & Federici, 2000 ) we report that Trichoplusia ni (Tn)GV helicase does not support replication of Autographa californica (Ac)MNPV in cells and larvae of T. ni. However, changes in only one or a few amino acids in the helicase (P143) of AcMNPV enable this virus to replicate in cells and larvae of Bombyx mori, where its replication is normally restricted (Maeda et al., 1993 ; Croizier et al., 1994 ; Kamita & Maeda, 1997 ; Argaud et al., 1998 ). In addition, P143 is required for replication of reporter plasmids in transient assays (Lu & Miller, 1995 ; Kool et al., 1995 ; Ahrens & Rohrmann, 1996 ; Heldens et al., 1997 ), results which support earlier studies suggesting that P143 is an essential component of the AcMNPV replisome (Lu & Carstens, 1991 ).

The classification of baculovirus helicases as DNA helicases is based on similarities between several conserved amino acid motifs (I, Ia, II–VI) of these proteins and those of enzymes that unwind duplex-DNA (Lu & Carstens, 1991 ; Gorbalenya et al., 1988 , 1989a , b ; Hodgeman, 1988 ; Matson & Kaiser-Rogers, 1990 ; Linder et al., 1989 ; Saraste et al., 1990 ; Walker et al., 1982 ). AcMNPV P143 has been shown to bind DNA non-specifically (Laufs et al., 1997 ). However, the two most important characteristics of the DNA helicases, the ability to hydrolyse ATP and unwind duplex DNA (Matson & Kaiser-Rogers, 1990 ), have not been demonstrated for any baculovirus helicase.

The objective of the present study was to determine whether the putative DNA helicase (P137; Bideshi et al., 1998 ) encoded by TnGV had ATPase or DNA binding and unwinding activities. In the present study we show that a recombinant P137 (rP137) has an intrinsic DNA-independent ATPase activity, an enzymatic function associated with helicase motifs I and II (Hodgeman, 1988 ; Linder et al., 1989 ; Matson & Kaiser-Rogers, 1990 ; Gorbalenya et al., 1988 ). However, we were unable to demonstrate either DNA binding or unwinding activity for TnGV P137.

To study TnGV P137, the gene (p137) encoding this putative helicase was hyper-expressed in BTI-TN-5B1-4 cells (Invitrogen) using the Bac-to-Bac HT AcMNPV Baculovirus Expression (Bacmid) system (Gibco BRL). To facilitate purification, codons for a histidine tag were added to the 5’ end of the gene. The histidine-tagged recombinant P137 protein (rP137) was purified by column chromatography using non-denaturing imidazole-based buffers and nickel–nitrilotriacetic acid (Ni–NTA) resin (Gibco BRL). After purification, two proteins (Fig. 1), a 140 kDa protein (rP137 including the histidine tag) and a 137 kDa protein (rP137 protein from which the histidine tag had been removed by cleavage with rTEv protease from Gibco BRL), were dialysed at 4 °C in 500 ml of TED buffer (10 mM Tris–HCl, pH 7·0, 1·0 mM EDTA, 1 mM DTT) and stored at -70 °C in storage buffer (10 mM Tris–HCl, pH 7·0, 2 mM EDTA, 2 mM DTT, 5 mM NaCl, 5 mM KCl, 20% glycerol). A 100 ng sample of each preparation was used in ATPase and helicase assays (Pause et al., 1993 ; Matson & Kaiser-Rodgers, 1990 ). Removal of the histidine tag with the rTEv protease, and incubation of rP137 at 37 °C for at least 30 min before performing the ATPase assay, significantly lowered enzymatic activity (data not shown). Therefore, the results reported here are based on assays performed with the histidine-tagged P137 (rP137).



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Fig. 1. Purified recombinant helicase (rP137) of TnGV. The rTEv protease-treated (+) and untreated (-) samples were subjected to electrophoresis to demonstrate purity in a 12% SDS–polyacrylamide gel stained with silver.

 
Like all DNA helicases, the ATPase activity of rP137 required Mg2+ as a co-factor, with optimal enzymatic activity occurring in the presence of 5 mM Mg2+ (Fig. 2B). However, unlike many DNA helicases (Matson & Kaiser-Rogers, 1990 ), rP137 activity did not require the presence of DNA in the assay (Fig. 2). Furthermore, ATPase activity was inhibited substantially in the presence of 10 ng of TnGV DNA. The maximal ATPase activity of rP137, for example, was 8-fold greater in the absence of TnGV DNA (1·17x105 versus 1·48x104 c.p.m.; Fig. 2B). A similar result was observed when the assay was performed with 10 ng of phage {lambda} DNA (Fig. 2B).



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Fig. 2. ATPase activity of the recombinant TnGV helicase, rP137. (A) Results of assays performed at 37 °C with 2·5 mM magnesium acetate in the presence (+) or absence (-) of TnGV rP137. (B) Results of assays with different concentrations of magnesium acetate and 10 ng of TnGV or {lambda} DNAs.

 
The DNA-independent ATPase activity of rP137 has several possible explanations. The conformation of this enzyme may have been altered by either the presence of the histidine tag or the method of purification, thereby exposing the catalytic site for ATP hydrolysis. Alternatively, the activity could be an intrinsic property of rP137. There is precedence for this in that at least two DNA helicases are known which exhibit DNA-independent ATPase activity: bovine papillomavirus type 1 (BPV-1) E1 and simian virus (SV) 40 large T antigen. The BPV-1 E1 protein requires ATP to unwind DNA (Seo et al., 1993a , b ), but exhibits ATPase activity in the absence of DNA (Santucci et al., 1995 ; Bream et al., 1993 ; Hughes & Romanos, 1993 ). The multifunctional SV40 large T antigen hydrolyses ATP in the absence of DNA, and though this activity is enhanced 5-fold in the presence of poly(dT), other nucleic acids have a minimal effect (Giachero & Hagar, 1979 ; Clark et al., 1981 ).

We do not know why the ATPase activity of rP137 decreased substantially in the presence of TnGV and {lambda} DNAs (Fig. 2B). One possibility is that the decreased activity was due to the sequestration of Mg2+ by viral DNA. Another possible explanation is that rP137 binds DNA, but upon binding, additional host- or TnGV-encoded protein(s) not present in our assay were required for efficient enzymatic activity. Although we demonstrated ATPase activity as a property of TnGV rP137, we were unable to show that this enzyme bound DNA or unwound partial duplex-DNA substrates. Therefore, as suggested for P143 (Laufs et al., 1997 ), it is possible that additional proteins encoded by TnGV or T. ni are required for rP137 full helicase activity.


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Received 12 January 2000; accepted 3 March 2000.