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Cap-independent translation of picornavirus RNAs: structure and function of the internal ribosomal entry site
S K Jang1, T V Pestova, C U Hellen
1Department of Microbiology, School of Medicine, State University of New York, Stony Brook.
Abstract:
Picornaviruses are mammalian plus-strand RNA viruses whose genomes serve as mRNA. A study of the structure and function of these viral mRNAs has revealed differences among them in events leading to the initiation of protein synthesis. A large segment of the 5' nontranslated region, approximately 400 nucleotides in length, promotes 'internal' entry of ribosomes independent of the non-capped 5' end of the mRNA. This segment, which we have called the internal ribosome entry site (IRES), maps approximately 200 nt down-stream from the 5' end and is highly structured. IRES elements of different picornaviruses, although functionally similar in vitro and in vivo, are not identical in sequence or structure. However, IRES elements of the genera entero- and rhinoviruses, on the one hand, and cardio- and aphthoviruses, on the other hand, reveal similarities corresponding to phylogenetic kinship. All IRES elements contain a conserved Yn-Xm-AUG unit (Y, pyrimidine; X, nucleotide) which appears essential for IRES function. The IRES elements of cardio-, entero- and aphthoviruses bind a cellular protein, p57. In the case of cardioviruses, the interaction between a specific stem-loop of the IREs is essential for translation in vitro. The IRES elements of entero- and cardioviruses also bind the cellular protein, p52, but the significance of this interaction remains to be shown. The function of p57 or p52 in cellular metabolism is unknown. Since picornaviral IRES elements function in vivo in the absence of any viral gene products, we speculate that IRES-like elements may also occur in specific cellular mRNAs releasing them from cap-dependent translation. IRES elements are useful tools in the construction of high yield expression vectors, or for tagging cellular genetic elements.
Insights
Picornaviruses use a special RNA structure called an internal ribosome entry site (IRES) for protein synthesis initiation. This IRES element allows translation without a cap, offering insights into viral gene expression and potential applications in biotechnology.
Area of Science:
- Molecular Biology
- Virology
- Biochemistry
Background:
- Picornaviruses are plus-strand RNA viruses that utilize their genomes as messenger RNA (mRNA) for protein synthesis.
- Protein synthesis initiation in eukaryotes typically relies on a 5' cap structure on the mRNA.
- Viral mRNAs of picornaviruses exhibit unique mechanisms for initiating translation.
Purpose of the Study:
- To investigate the structure and function of picornaviral mRNAs, focusing on the mechanism of protein synthesis initiation.
- To identify and characterize the novel RNA element responsible for cap-independent translation.
- To explore the potential biological and biotechnological implications of this RNA element.
Main Methods:
- Analysis of picornaviral mRNA sequences and structures.
- In vitro and in vivo studies of translation initiation.
- Identification and characterization of RNA-protein interactions using techniques like protein binding assays.
- Comparative analysis of IRES elements across different picornavirus genera.
Main Results:
- A highly structured RNA segment, termed the internal ribosome entry site (IRES), located in the 5' non-translated region, facilitates cap-independent ribosome entry and translation initiation.
- IRES elements vary in sequence and structure among picornaviruses but share conserved functional motifs, including a Yn-Xm-AUG unit essential for function.
- Specific cellular proteins, p57 and p52, bind to certain IRES elements, with p57 interaction being crucial for cardiovirus translation.
- IRES elements function independently of viral proteins, suggesting potential roles in cellular mRNA translation.
Conclusions:
- Picornaviral IRES elements represent a distinct mechanism for initiating protein synthesis, bypassing the conventional cap-dependent pathway.
- Sequence and structural similarities in IRES elements correlate with the phylogenetic relationships of picornavirus genera.
- The interaction with cellular proteins like p57 is vital for IRES-mediated translation, though their broader cellular roles remain to be elucidated.
- The discovery of IRES elements opens possibilities for their use in expression vectors and for understanding cap-independent translation in cellular systems.