Alternate, virus-induced membrane rearrangements support positive-strand RNA virus genome replication

All positive-strand RNA [(+)RNA] viruses replicate their RNA on intracellular membranes, often in association with spherular invaginations of the target membrane. For brome mosaic virus, we previously showed that such spherules serve as compartments or mini-organelles for RNA replication and that their assembly, structure, and function have similarities to the replicative cores of retrovirus and double-stranded RNA virus virions. Some other (+)RNA viruses conduct RNA replication in association with individual or clustered double-membrane vesicles, appressed double membranes, or other structures whose possible relationships to the spherular invaginations are unclear. Here we show that modulating the relative levels and interactions of brome mosaic virus replication factors la and 2a polymerase (2a(pol)) shifted the membrane rearrangements associated with RNA replication from small invaginated spherules to large, karmellae-like, multilayer stacks of appressed double membranes that supported RNA replication as efficiently as spherules. Spherules were induced by expressing 1 a, which has functional similarities to retrovirus virion protein Gag, or 1 a plus low levels of 2a(pol). Double-membrane layers were induced by la plus higher levels of 2a(pol) and were suppressed by deleting the major la-interacting domain from 2a(pol). The stacked, double-membrane layers alternated with spaces that, like spherule interiors, were 50-60 nm wide, connected to the cytoplasm, and contained la and 2a(pol). These and other results suggest that seemingly diverse membrane rearrangements associated with RNA replication by varied (+)RNA viruses may represent topologically and functionally related structures formed by similar protein-protein and protein-membrane interactions and interconverted by altering the balances among those interactions.