Ultrastructural Location of Non-structural Protein 3A of Cucumber Mosaic Virus in Infected Tissue Using Monoclonal Antibodies to a Cloned Chimeric Fusion Protein

Summary Cloned DNA representing the sequence coding for the non-structural protein 3A from cucumber mosaic virus (CMV) was inserted in an expression vector containing a truncated portion of the Protein A gene from Staphylococcus aureus. Expressed fusion protein was purified from Escherichia coli cell extracts by affinity chromatography on rabbit gamma globulin-conjugated Sepharose and used as an immunogen for the production of monoclonal antibodies (MAbs). Five MAbs that reacted with pXCM3A13 fusion protein in solid phase ELISA were able to immunoprecipitate specifically protein 3A from mixtures of in vitro translation products of CMV RNA. None of these antibodies reacted with the analogous protein in translation products of brome mosaic virus RNA, but all of them reacted with the 3A protein from tomato aspermy virus. Immunogold labelling of ultrathin sections of CMV-infected tobacco tissue with MAb 3H12 demonstrated that the 3A protein accumulated within the nucleoli of these cells. This location differs from that of the analogous 3A protein of alfalfa mosaic virus which is associated with the middle lamella of the cell walls of infected tobacco cells.