Novel NTP Binding Property of Rice Dwarf Phytoreovirus Minor Core Protein P5

AbstractRice 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-PPi, dGTP-PPi, ATP-PPi, and UTP-PPiexchange reactions. In SDS–polyacrylamide gel electrophoresis, the 89-kDa protein comigrated with the S5-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 (