Rice dwarf phytoreovirus (RDV) mRNA synthesized from purified virion has a cap structure, m7GpppAm-, which suggests the presence of guanylyltransferase activity in the virion. We attempted to identify the enzyme involved in the cap formation by using a nucleoside triphosphate binding assay. Incubation of virion with [α-32P]GTP resulted in labeling of an 89-kDa protein that had not previously been identified in purified virus preparations. Interestingly this protein also covalently bound UTP and ATP, which is not a property of the known guanylyltransferases. RDV particles catalyzed GTP-PP(i), dGTP-PP(i), ATP-PP(i), and UTP-PP(i) exchange reactions. In SDS-polyacrylamide gel electrophoresis, the 89-kDa protein comigrated with the SE-coded protein, P5, which had been expressed by a baculovirus vector. Moreover, the labeled 89-kDa protein was precipitated by an antiserum against this recombinant RDV P5. Careful reinvestigation of purified virus particles by SDS-polyacrylamide gel electrophoresis and Western blotting analyses showed that they contained a small amount of P5 (<0.5% of the total protein) within the core. These results may suggest that the minor core protein of RDV, which is coded by S5, is a candidate guanylyltransferase, although the biological significance of its ATP and UTP binding activities remains largely unknown.
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