Poliovirus (also called human enterovirus C), the causative agent of poliomyelitis, has caused disease in humans for centuries (see Historical Highlights). It primarily targets children, with paralysis being the tragic result in some cases.
The life cycle of poliovirus illustrates another strategy used by RNA viruses to ensure synthesis of needed proteins: synthesis of a polyprotein. Poliomyelitis (section)
The biology of polioviruses (family Picornaviridae) has been intensely studied for decades. But many questions linger. This nonenveloped virus makes its way into a human host by ingestion. It attaches to a cell surface molecule called human PV receptor (PV for poliovirus).
The nucleocapsid enters the host cell, and the plus-strand RNA genome is released into the cytosol while the virion is at the cell periphery and held within an endocytic vesicle (figure 27.23). The genome acts as mRNA and is translated by host cell ribosomes.
This process has garnered considerable attention because the virus’s RNA does not have the 59 cap found on eukaryotic mRNAs, which is important for ribosome binding. Poliovirus “tricks” its host into translating its capless RNA using a 59 region on the RNA called the internal ribosome entry site (IRES).
In this region the ssRNA folds back on itself and forms extensive secondary structures (regions of dsRNA and numerous ssRNA loops), which are important for recognition of the RNA by ribosomes (see figure)
Translation of The poliovirus genome yields a single polyprotein (see figure). A polyprotein is a large protein that can be cleaved into smaller proteins by enzymes called proteases. The poliovirus polyprotein has protease activity and cleaves itself into three smaller proteins.
Additional cuts in these proteins eventually yield all the structural proteins needed for capsid formation, as well as the virus’s RNA-dependent RNA polymerase. The polymerase is used to generate negative-strand RNA molecules that serve as templates for synthesis of plus-strand RNAs.
Proteins VP1-4 are structural proteins. Protein 3D is the RNA-dependent RNA polymerase.
Some of these are translated, but eventually, most will be used as new genomes and incorporated into capsids. All of the synthetic activity occurs in a replication complex. Mature particles are released from the cell by lysis. Synthesis stage