If there is one truism about picornaviruses, it is that the entire replication cycle of these simple positive-strand RNA viruses takes place in the cytosol. This statement is usually made to directly contrast picornaviruses with retroviruses or DNA viruses that require transport to the nucleus. However, the statement is meant quite literally. Some enveloped RNA viruses enter organelles to bud from the cellular secretion pathway, while other RNA viruses replicate their genomes in tightly controlled organelle invaginations. In contrast, every step in the replication of picornaviruses, once the genome has entered the cytosol, has long been thought to take place directly in the cytoplasm or on the cytoplasmic face of membranous structures
The role of autophagosomes in poliovirus replication has long been controversial. Some believe the cytoplasmic face of these vesicles to be a site of virus RNA replication. This was primarily due to the localization of multiple virus-encoded RNA replication proteins to the autophagosome membrane. A competing hypothesis emerged observes that viral RNA replication proteins localized to single-membraned vesicles containing components of the cellular COPII machinery.
But the COPII and autophagy hypotheses might not be mutually exclusive. Single-membraned vesicles predominate in the first few hours of poliovirus infection. Later, convoluted invaginations of the single-membraned vesicles are observed. This results in structures morphologically similar to the crescent-shaped phagophore, which is the precursor to the double-membraned autophagosome. By 6 hours post-infection, double-membraned vesicles predominate. Viral proteins and active RNA replication is associated with both types of structure. However, the exponential phase of RNA replication occurs when predominantly single-membraned vesicles are present. The authors proposed a model in which single-membraned vesicles morph into double-membraned vesicles, and suggested that the single-membraned vesicles are the primary sites of viral genome replication.
Picornaviruses are among the simplest human viruses, physically consisting of a positive-sense RNA genome and a capsid. The current model for exit of picornaviruses from cells is disruption of the plasma membrane resulting in a lysis event that releases waiting cytoplasmic virions. However, if a cell full of virus-containing double-membraned vesicles lyses, releasing the vesicles, then two lipid bilayers remain between the virions and the receptors on the surface of the next cell.
This new model leaves us with a new paradigm for picornavirus replication. These viruses, for so long thought to be cytoplasmic, may in fact be more infectious if engulfed in an organelle lumen. These so-called “naked viruses,” thought to be bare in the cytoplasm, may in fact swaddle themselves in multiple layers of membranes prior to cell lysis. This work may reveal a replication strategy that can provide a mechanistic evolutionary link between the enveloped and nonenveloped viruses.