Replication of all known positive-strand RNA viruses occurs in replication complexes associated with intracellular membranes. The putative nucleoside triphosphate binding (NTB) protein of Tomato ringspot virus (ToRSV) contains a stretch of hydrophobic residues at its C terminus, suggesting that it may act as a membrane anchor for the replication complex. Anti-NTB antibodies detected two predominant proteins in membrane-enriched fractions (the 66-kDa NTB and 69-kDa NTB-VPg proteins) along with other, larger proteins. The proteins containing the NTB domain cofractionated with markers of the endoplasmic reticulum (ER) and with ToRSV-specific RNA-dependent RNA polymerase activity in sucrose gradients. ToRSV infection induced severe changes in the morphology of the ER in plants expressing an ER-targeted green fluorescent protein (ER-GFP), and proteins containing the NTB domain colocalized with ER-GFP in indirect immunofluorescence assays. The proteins containing the NTB domain have properties of integral membrane proteins. Proteinase K protection assays using purified membranes from infected plants revealed that although the central portion of the NTB domain is exposed to the cytoplasmic face of the membranes, an 8-kDa fragment, recognized by anti-VPg antibodies, is protected by the membranes. This fragment probably consists of the 3-kDa VPg and the 5-kDa stretch of hydrophobic residues at the C terminus of the NTB protein, suggesting a luminal location for the VPg in at least a portion of the molecules. These results provide evidence that proteins containing the NTB domain are transmembrane proteins associated with ER-derived membranes and support the hypothesis that one or several of the proteins containing the NTB domain anchor the replication complex to the ER.Replication of the genomes of all characterized positivestrand RNA viruses occurs in large complexes which are associated with intracellular membranes (5). Many positive-strand RNA viruses induce extensive proliferation and modification of intracellular membranes in their hosts, which often results in the accumulation of numerous membranous vesicles. Doublestranded RNA (dsRNA) replication intermediates and viral replication factors are associated with the membranous vesicles, which are thought to be the site of the replication (6,7,18,25,48,49,52). The requirement for intact membranes for successful virus replication has also been demonstrated using cell-free replication systems (2, 35, 59). Different viruses associate with and modify different types of intracellular membranes (5). Although the importance of the association of viral replication factors with intracellular membranes is well documented, the mechanisms by which replication complexes are fixed on specific membrane sites remain poorly understood.Picornaviruses and several plant viruses with similar genomic organization have been shown to replicate in association with membranes derived from the endoplasmic reticulum (ER) (6,42,47,51). One or several viral proteins are thought to act as membrane an...