A study was made of the proteins synthesized in cowpea protoplasts infected with cowpea mosaic virus (CPMV). Protoplasts were labelled with S-methionine at different times during the infection cycle and the proteins synthesized were analysed by SDS-polyacrylamide slab gel electrophoresis of different subcellular fractions. Since no shut-off of host protein synthesis occurred, virus-specific protein synthesis had to be distinguished from a high background of protoplast proteins. Eleven polypeptides with mol. wt. of 170, 130, 112, 110, 87, 84, 68, 37, 30, 24 and 23 (all × 10) were detected whose synthesis was either induced or stimulated by CPMV infection. No differences were observed between the electrophoretic patterns of samples from uninfected and CPMV-infected protoplasts during the first 8 h of infection (latent phase). The 170 × 10 and 30 × 10 species were the first virus-related polypeptides detectable by labelling between 9 and 15 h after inoculation. From about 16 h after inoculation, all other virus-related proteins appeared to be synthesized at increasing rates. The polypeptides with mol. wt. of 170, 130, 112, 110, 68, 37 and 23 (all × 10) could be designated as virus-induced; the others may well be host-coded proteins, the synthesis of which is stimulated by CPMV infection. Although CPMV replication is known to occur in close association with virus-induced cytopathic membrane structures the 112 × 10 and 68 × 10 mol. wt. polypeptides were the only virus-related proteins specifically found in the particulate fractions of the protoplasts. From the 11 virus-related proteins, the 37 × 10, 24 × 10 and 23 × 10 mol. wt. proteins were identified as virus coat proteins. The 170 × 10 and 30 × 10 mol. wt. polypeptides also appeared to be virus-specific and to be coded for by CPMV bottom component RNA.


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