Synthesis of Sonchus Yellow Net Virus Proteins in Infected Cowpea Protoplasts

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Summary We studied protein synthesis in cowpea protoplasts infected with Sonchus yellow net virus (SYNV). The structural viral proteins G, N, M1 and M2 were identified by immunoprecipitation. In addition, four high molecular weight proteins and two proteins with molecular weights of 52000 (p52) and 38000 (p38) were precipitated. One of the four high molecular weight proteins may represent the L protein; the others are possibly aggregates of the G protein or its unglycosylated form. The nature of proteins p52 and p38 is unclear. Structural viral proteins first appeared between 8 h and 12 h after inoculation. The G protein is glycosylated by N-glycoside-linked sugar residues. Glycosylation of the G protein was prevented by incubation of the protoplasts in 10 µg/ml tunicamycin. A protein, co-migrating with the M1 protein, was labelled with [32P]orthophosphate in protoplasts infected with SYNV, but not in controls of mock-inoculated protoplasts or protoplasts inoculated with cowpea mosaic virus.