In this article we will discuss about the structure of picornavirus.
Each capsid is composed of 60 copies of four structural proteins of which each proto-meter consists of four polypeptides known as VP1, VP2, VP3 and PV4. VP1-VP3 are exposed on the virion surface, while VP4 lies buried in close association with the RNA core (Fig. 17.21).
All the VP polypeptides originate from one protomer known as VPO that is cleaved to give the different capsid components. Immunogenic sites are located on the exposed external parts of the capsid.
All the subgroups are structurally identical. They have icosahedral capsid of 27-30 nm which is composed of 32 capsomers. The length of viral genome is about 2500 nm; therefore it is tightly packaged within the capsid along with substances such as sodium ions which cancels out the negative charges on RNA caused by the phosphate groups.
The uptake of un-enveloped viruses always considered to involve endocytosis into an endosome i.e. it is pH-dependent and sensitive to ionophores and lysosomotropic agents. Five VP1 protein subunits at the 5 fold axis of symmetry form a canyon in the capsid which is the recognition site for the receptor.
The polio receptor having an amino terminal Ig-like domain (like CD4) binds to the virus, a trans-membrane domain and a cytoplasmic domain (Fig. 17.22). There are many thousand copies of receptor present on a cell.
The virus is endocytosed into an endosome after binding to the receptor. At a low pH, poliovirus becomes more lipophilic and associate with the endosome membrane possibly forming a pore. Presumably the pH shift results in exposure of hydrophobic domains in the capsid proteins. During this process VP4 is lost from the particle and VP2 may also be lost.
The genomic RNA is ejected through an endosomal pore into the cytoplasm (Fig. 17.23). Poliovirus can also enter the cytoplasm directly in a pH-independent process. After adherence to the receptor, the virus can be eluted again. But if it happens, the particle undergoes conformational changes due to the loss of VP4. This causes loss of infectivity.
The genome of picornavirus is mono-partite, linear, (+) sense ssRNA, 7.1- 8.9 kb long, polyadenylated consisting of a single open reading frame (ORF) that encode a polyprotein (Fig. 17.24). Viral genomic RNA has a viral protein (VPg) at its 5′ end instead of a methylated nucleotide cap structure.
The long UTR at the 5′ end contains an internal ribosome entry site (IRES). The PI region encodes the structural polypeptides. The P2 and P3 regions encode the non-structural proteins associated with replication.
The shorter 3′ UTR is important in (-) strand synthesis. L is an additional N-terminal leader protein encoded only by aphthoviruses. Genomic RNA is translated directly by polysomes. Un-translation regions (UTR) are located at both the ends of genome. The (+) ssRNA genome consists of poly A tail at 3′ end.