Processing of VPg-containing polyproteins encoded by the B-RNA from cowpea mosaic virus.

To study the processing of putative VPg precursors the expression of specific mutant transcripts derived from a full-length cDNA clone of cowpea mosaic virus (CPMV) B-RNA was examined in a rabbit reticulocyte lysate system. This study revealed that the 170K protein produced by a B-RNA mutant that lacks the 32K coding region was efficiently processed by mainly intramolecular cleavages at three different sites into three sets of proteins of 60K + 110K, 84K + 87K, and 58K + 112K. Further cleavage of the 60K protein into 58K and VPg has not been observed in this in vitro system. The 84K protein can be further processed by an intramolecular cleavage reaction via two alternative pathways, either into 26K (VPg + 24K) and 58K proteins or into 24K and 60K proteins. VPg can be released from the 112K (VPg + 110K) precursor either directly or via the 26K intermediate. Immunoblot analysis showed that the 112K protein is present in CPMV-infected plant cells indicating that the in vitro observations may hold true in vivo.