Picornavirus mechanism

The structures of the picornaviruses (native, with receptor bound, in the presence of acid, with a myriad of compounds bound, and of acid- and drug-resistant mutants) have yielded valuable information about possible molecular mechanisms for their uncoating. These same studies have suggested the mechanism by which these uncoating inhibitors work. A by-product of this research is the hypothesis that these compounds may mimic naturally occurring factors that occupy the VP1 pocket. The hunt for these natural compounds and their significance is underway. 

The mechanisms by which ICAM-1 binding to HRV triggers virus destabilization and uncoating are not understood. A hydrophobic pocket inside VPl lies directly beneath the canyon floor. This pocket was shown to be the binding site for certain antiviral compounds that inhibit the replication of HRVs and related picornaviruses (Fox et al., 1986 Smith et al., 1986). Experimental evidence has shown that antiviral drugs bound to this hydrophobic pocket lock the viral particle in a state in which neither VP4 nor the N-terminus of VPl can be externalized (Lewis et al., 1998), thus preventing uncoating. 

Hepatitis B virus (HBV) is widespread throughout human populations, specially in Asia and Africa, and it has been estimated that over 200 million carriers exist, some of whom are eventually expected to develop liver carcinoma or cirrhosis. HBV shows a strict tropism for liver hepatocytes in which it displays a protected replication with resultant foci of liver necrosis. The virus is a member of the Hepadnaviridae, along with several other species, and it replicates by a mechanism which appears to be unique to this family. In contrast, hepatitis A virus is a picornavirus and the hepatitis D agent appears to be a viroid-like RNA enclosed within a hepatitis B capsid, and consequently depends upon its association with the HBV for its spread and survival. Control may be effected by passive immunization (with hyperimmune globulin) or by various types of vaccines which are currently being developed and improved. Specific chemotherapy has not been consistently successful, but in some countries (e g., India and China), plant extracts have provided some success. 

In this review I have outlined several theories that have been proposed to explain the mechanism by which picornaviruses inhibit cellular protein synthesis. Some theories seem less likely than others. Inhibition by ds ENA, for example, is no longer thought to be a likely possibility. In cell-free extracts ds ENA inhibits both cellular and viral mRNA translation (61). The inhibitor of cellular protein synthesis would be expected to be selective in its inhibitory activity. It is also apparent that picornavirus infection does not result in the degradation or alteration of cellular mENA (9> 27, 29 51). So,.too, experiments demonstrating that protein synthesis inhibition takes place in the absence of significant viral ENA synthesis (I4) tend to weaken the argument that protein synthesis inhibition results from direct competition of viral mENA with cellular ENA for initiation factor eIE-4D (47) As mentioned earlier, superinfection with poliovirus of cells infected with VSV prevents VSY mENA translation (J2, 56). In lysates from uninfected HeLa cells, however, 7SY mENA translation is favored over poliovirus mENA translation when both mENA species are present in equimolar saturating concentrations (55) If competition were a major cause of cellular protein synthesis inhibition, one would have expected poliovirus mENA to out-compete VSV mENA in cell-free translation, not the contrary. 

The process of replication of the viral RNA is probably the most extensively studied aspect of the molecular biology of picornaviruses. Beginning (just to set a date) in the early sixties with the reports by Sanders (l) and Darnell (2) on the time-course of RNA synthesis, experimental data have accumulated at such a pace that an attempt to provide the reader with a more or less complete list of references would prove not only impossible (because of the mavoidable omission of important contributions), but would also be out of the scope of this chapter. The purpose of this presentation being to review our current understanding of the mechanism of RNA synthesis in picornavirus-infected cells, I shall only refer to those few papers strictly pertinent to the problems inder consideration. 

It was soon realized, however, that some kind of regulatory mechanism must intervene, because the synthesis of virus-induced RNAs was shown to be quite an asymmetrical process The bulk of the RNA found in the cytoplasm of picornavirus-infected cells is virion-like (i.e. "plus" strand), and only a very minor fraction of the newly synthesized RNA would hybridize to the RNA extracted from virions. Several explanations of this phenomenon have been offered ... 

The replication complex can be isolated with the particulate fraction of the cytoplasm of picornavirus-infected cells (71) Under proper conditions the ECs are able to continue vitro the synthesis of ENA, providing a unique tool to study the mechanism of ENA synthesis with all the advantages (and all the limitations, too) of an m vitro cell-free system. 

The above sections have described several different mechanisms which have been proposed and explored during the last decade to explain the selective inhibition of host cell protein synthesis in poliovirus-infected cells. Admittedly, this author s bias has presented each mechanism as a straw man, requiring the reader to await what is perceived at this time to be the correct explanation for this aspect of the regulation of protein synthesis in poliovirus-infected cells. The favored model will be discussed in this and subsequent sections. It is important to state, however, that there is no convincing evidence that other picornaviruses are necessarily similar to poliovirus in the mechanism(s) utilized for host protein synthesis inhibition and that the mechanisms described above, as well as others, cannot all be dismissed in every case of picorna virus-induced protein synthesis inhibition. Thus, the data for other picornaviruses will be reviewed separately. 

Many lytic viruses, other than picornaviruses, markedly inhibit host cell protein synthesis during the course of the infectious cycle. None have been investigated to the same extent as poliovirus with respect to the mechanism of this function. However, there are a few preliminary studies which might be interpreted as indications of similar effects on initiation factor activity. 

Such a profound inhibition of cellular protein synthesis is not an uncommon feature of virus infections of mammalian cells. Several lines of research have recently converged to demonstrate that adenoviruses, like certain other viruses that inhibit cellular protein synthesis, such as the picornaviruses poliomyelitis virus and encephal-omyocarditis virus, have evolved a mechanism to permit the selective translation of viral mRNA species. Although both adenoviruses and picornaviruses can efficiently redirect the translational machinery of their host cells, the actual molecular mechanisms employed appear to be quite distinct. 

The initial observations by Enders (1954) and his collaborators that virus-cell interaction culminates in the destruction of the infected cells in culture made it possible to study these alterations at both the morphological and biochemical levels. Highly cytocidal viruses, during the process of either productive or abortive virus replication, ultimately kill the cell. Notable examples of this type of viruses are the picornaviruses, the rhabdoviruses, the herpesviruses, and the poxviruses. Early studies of virus cytopathology were mainly descriptive in nature, documenting the various types of virus-induced morphological alterations to cells. With the advent of new biochemical techniques, it became possible to scrutinize the mechanisms by which these viruses altered the infected cells. The main objective of these studies has been to define the initial step of virus-induced alteration... 

Certain eukaryotic RNA viruses (e.g., picornaviruses) contain a single-stranded genomic RNA (—7.5 kb) that functions as a polycistronic mRNA. The 5 ends of the picornaviral RNAs are not capped, but are covalently linked to a small viral protein, VPg. In contrast to the average of 55-nt-Iong 5 UTR in yeast and higher eukaryotic mRNAs, the 5 UTRs of picornaviruses are over 700 bases long and contain extensive stem—loop structures. The particular 5 UTR structures of picornaviruses by no means fit the widely accepted scanning mechanism of... 

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