Identification and Characterization of the 480-Kilodalton Template-Specific RNA-Dependent RNA Polymerase Complex of Red Clover Necrotic Mosaic Virus

Replication of positive-strand RNA viruses occurs through the assembly of membrane-associated viral RNA replication complexes that include viral replicase proteins, viral RNA templates, and host proteins. Red clover necrotic mosaic virus (RCNMV) is a positive-strand RNA plant virus with a genome consisting of RNA1 and RNA2. The two proteins encoded by RNA1, a 27-kDa protein (p27) and an 88-kDa protein containing an RNA-dependent RNA polymerase (RdRP) motif (p88), are essential for RCNMV RNA replication. To analyze RCNMV RNA replication complexes, we used blue-native polyacrylamide gel electrophoresis (BN/PAGE), which enabled us to analyze detergent-solubilized large membrane protein complexes. p27 and p88 formed a complex of 480 kDa in RCNMV-infected plants. As a result of sucrose gradient sedimentation, the 480-kDa complex cofractionated with both endogenous template-bound and exogenous template-dependent RdRP activities. The amount of the 480-kDa complex corresponded to the activity of exogenous template-dependent RdRP, which produced RNA fragments by specifically recognizing the 3′-terminal core promoter sequences of RCNMV RNAs, but did not correspond to the activity of endogenous template-bound RdRP, which produced genome-sized RNAs without the addition of RNA templates. These results suggest that the 480-kDa complex contributes to template-dependent RdRP activities. We subjected those RdRP complexes to affinity purification and analyzed their components using two-dimensional BN/sodium dodecyl sulfate-PAGE (BN/SDS-PAGE) and mass spectrometry. The 480-kDa complex contained p27, p88, and possible host proteins, and the original affinity-purified RdRP preparation contained HSP70, HSP90, and several ribosomal proteins that were not detected in the 480-kDa complex. A model for the formation of RCNMV RNA replication complexes is proposed.