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Abstract
The intact virion of bluetongue virus comprises ten segments of dsRNA enclosed in two concentric protein capsids. The core, which is transcriptionally active, includes three minor proteins (VP1, VP4 and VP6) which are considered to be the candidates for the core-associated enzymes that transcribe and modify full-length mRNA copies for each of the ten genome segments. Using purified recombinant VP4 protein and core-like particles containing VP4, in this report it is demonstrated that VP4 has nucleoside triphosphatase (NTPase) activity. VP4 is a nonspecific NTPase that hydrolyses four types of ribo- nucleoside triphosphate (NTP) to the corresponding nucleoside diphosphate. The substrate preference was GTP ATP UTP CTP. NTP hydrolysis by VP4 was maximal when the Mg2 or Ca2 ion concentrations were 4 mM or 6 mM, respectively. The presence of single-stranded polynucleotides poly(A), poly(U) and poly(C) had little effect on the NTPase activity. Although the enzyme exhibited a broad temperature optimum around 40 °C, the pH optimum was sharp, between pH 7 5 and 8. The K m and V max of ATP hydrolysis were calculated to be 0·25 0·05 μM ATP and 55 4 pmol ATP hydrolysed min 1 μg \ respectively. The Km was affected by the addition of poly(A) to only a small extent in contrast to the Vmax, which was increased by at least twofold.
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