Protein synthesis in virus-infected plants: II. The synthesis and accumulation of TMV and TMV coat protein in subcellular fractions of TMV-infected tobacco

Abstract

Subcellular fractions of TMV-infected leaves, obtained after 2 hr of 3H-leucine incorporation after various times of infection or continuous 3H-leucine labeling for long periods after infection, were assayed for infectivity and, after denaturation with SDS, analyzed for total TMV coat protein by acrylamide gel electrophoresis. The nuclear and mitochondrial fractions contained little or no detectable infectivity and coat protein. The cytoplasmic fraction did not contain any viral infectivity, but with short labeling it contained a high proportion of the total labeled coat protein. After long, continuous labeling, the proportion of total labeled coat protein in the cytoplasm was low, indicating that most of the newly synthesized protein was used to coat viral RNA. Although the ribosomal fraction sedimenting at 105,000 g contained the majority of the infective virus, the chloroplast fraction appeared to be a site of virus storage since virus, increasing with time of infection, was associated with it. At early stages of infection, up to half of the total short labeled coat protein was present in the chloroplast fraction. This high proportion of protein decreased with time of infection, and with long labeling became low. Since the rapidly labeled protein was not found free, and was greatly in excess of the virus concentration, when compared to the ratio of labeled protein to virus in the mitochondrial or ribosomal fractions, it is postulated that the chloroplasts are a site of virus assembly.