. During viral infection, the 66K protein localizes to virus-induced chloroplastic membrane vesicles, which are closely associated with TYMV RNA replication. To investigate the determinants of its subcellular localization, the 66K protein was expressed in plant protoplasts from separate plasmids. Green fluorescent protein (GFP) fusion and immunofluorescence experiments demonstrated that the 66K protein displayed a cytoplasmic distribution when expressed individually but that it was relocated to the chloroplast periphery under conditions in which viral replication occurred. The 66K protein produced from an expression vector was functional in viral replication since it could transcomplement a defective replication template. Targeting of the 66K protein to the chloroplast envelope in the course of the viral infection appeared to be solely dependent on the expression of the 140K protein. Analysis of the subcellular localization of the 140K protein fused to GFP demonstrated that it is targeted to the chloroplast envelope in the absence of other viral factors and that it induces the clumping of the chloroplasts, one of the typical cytological effects of TYMV infection. These results suggests that the 140K protein is a key organizer of the assembly of the TYMV replication complexes and a major determinant for their chloroplastic localization and retention.A universal feature of eukaryotic positive-strand RNA viruses is that replication of their genomes is closely associated with intracellular membranes (reviewed in reference 8). Most purified viral RNA replication complexes copurify with membrane extracts from infected cells (reviewed in reference 10) and, although in some cases RNA synthesis activity can be solubilized (24, 67), in vivo and in vitro studies suggest that the presence of membranes and/or phospholipids is essential for at least some steps of RNA replication (37,41,67). It was proposed that these membranes can play both a structural and a functional role in the replication complex.Electron microscopy observations of infected cells revealed that many positive-stranded RNA viruses induce proliferation and/or reorganization of the intracellular membranes of their host to create a membrane compartment in which RNA replication takes place. Depending on the virus, a variety of membrane systems can be concerned, including the early and late endomembrane systems (52, 59), the nuclear envelope (13), the vacuole (64), the endosomes and lysosomes (17, 59), the peroxisomes (56), chloroplasts (35), and mitochondria (14, 40). The fact that distinct types of membranes are involved in the replication of different viruses suggests the establishment of specific interactions between such host membranes and virusencoded proteins. A number of viral proteins that target replication complexes to intracellular membranes have been identified (9,48,55,63). Membrane interaction of host-encoded factors that are part of the viral replication complex has also been reported (22,68).Despite this universal association of positive-strand ...

Turnip yellow mosaic virus (TYMV) encodes a 206-kDa (206K) polyprotein with domains of methyltransferase, proteinase, NTPase/helicase, and RNA-dependent RNA polymerase (RdRp). In vitro, the 206K protein has been shown to undergo proteolytic processing, giving rise to the synthesis of 140-kDa (140K) and 66-kDa (66K) proteins, the latter comprising the RdRp protein domain. Antibodies were raised against the 66K protein and were used to detect the corresponding viral protein in infected cells; both leaf tissues and protoplasts were examined. The antiserum specifically recognized a protein of approximately 66 kDa, indicating that the cleavage observed in vitro is also functional in vivo. The 66K protein accumulates transiently during protoplast infection and localizes to cellular membrane fractions. Indirect immunofluorescence assays and electron microscopy of immunogold-decorated ultrathin sections of infected leaf tissue using anti-66K-specific antibody revealed labeling of membrane vesicles located at the chloroplast envelope.