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Poly(A) binding protein 1 enhances cap-independent translation initiation of neurovirulence factor from avian herpesvirus.

Tahiri-Alaoui A, Zhao Y, Sadigh Y, Popplestone J, Kgosana L, Smith LP, Nair V - PLoS ONE (2014)

Bottom Line: We report a novel viral translational control strategy involving the recruitment of PABP1 to the 5' leader internal ribosome entry site (5L IRES) of an immediate-early (IE) bicistronic mRNA that encodes the neurovirulence protein (pp14) from the avian herpesvirus Marek's disease virus serotype 1 (MDV1).We provide evidence for the interaction between an internal poly(A) sequence within the 5L IRES and PABP1 which may occur concomitantly with the recruitment of PABP1 to the poly(A) tail.RNA interference and reverse genetic mutagenesis results show that a subset of virally encoded-microRNAs (miRNAs) targets the inhibitor of PABP1, known as paip2, and therefore plays an indirect role in PABP1 recruitment strategy by increasing the available pool of active PABP1.

View Article: PubMed Central - PubMed

Affiliation: The Pirbright Institute, Ash Road, Pirbright, Woking, Surrey, United Kingdom.

ABSTRACT
Poly(A) binding protein 1 (PABP1) plays a central role in mRNA translation and stability and is a target by many viruses in diverse manners. We report a novel viral translational control strategy involving the recruitment of PABP1 to the 5' leader internal ribosome entry site (5L IRES) of an immediate-early (IE) bicistronic mRNA that encodes the neurovirulence protein (pp14) from the avian herpesvirus Marek's disease virus serotype 1 (MDV1). We provide evidence for the interaction between an internal poly(A) sequence within the 5L IRES and PABP1 which may occur concomitantly with the recruitment of PABP1 to the poly(A) tail. RNA interference and reverse genetic mutagenesis results show that a subset of virally encoded-microRNAs (miRNAs) targets the inhibitor of PABP1, known as paip2, and therefore plays an indirect role in PABP1 recruitment strategy by increasing the available pool of active PABP1. We propose a model that may offer a mechanistic explanation for the cap-independent enhancement of the activity of the 5L IRES by recruitment of a bona fide initiation protein to the 5' end of the message and that is, from the affinity binding data, still compatible with the formation of 'closed loop' structure of mRNA.

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Model depicting the closed loop topology for the bicistronic immediate-early transcript (IE) that encodes RLORF9 and the pp14 from MDV1.In this model, only the pp14b isoform is shown, which is under the translation control of the 5L IRES. The internal poly(A) of the 5L IRES recruits PABP1 to the 5' end of the mRNA, which may be concomitant with the recruitment of PABP1 to the poly(A) tail of the message, leading to circularization of the bicistronic dual IRES IE-mRNA. A subset of viral miRNAs down-regulate the expression level of paip2 which is a well-known inhibitor of PABP1. This down regulation of paip2 indirectly contributes to an increased level of the available pool of active PABP1 which interacts with the internal poly(A) sequence of the 5L IRES hence leading to increased IRES activity.
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pone-0114466-g008: Model depicting the closed loop topology for the bicistronic immediate-early transcript (IE) that encodes RLORF9 and the pp14 from MDV1.In this model, only the pp14b isoform is shown, which is under the translation control of the 5L IRES. The internal poly(A) of the 5L IRES recruits PABP1 to the 5' end of the mRNA, which may be concomitant with the recruitment of PABP1 to the poly(A) tail of the message, leading to circularization of the bicistronic dual IRES IE-mRNA. A subset of viral miRNAs down-regulate the expression level of paip2 which is a well-known inhibitor of PABP1. This down regulation of paip2 indirectly contributes to an increased level of the available pool of active PABP1 which interacts with the internal poly(A) sequence of the 5L IRES hence leading to increased IRES activity.

Mentions: As a nuclear DNA virus, MDV1 produces capped and polyadenylated mRNAs that are indistinguishable from host mRNAs. Therefore, MDV1 has to compete with the host for resources required for mRNA translation. An elegant and detailed account of how viruses gain control of key cellular signalling pathways and subvert the host protein synthesis machinery was previously reviewed [1], [2]. The temporal gene expression of MDV1 suggests that IE and late gene expression might use different strategies of translation control so that viral protein synthesis ensues with minimal disruption to the host. This is critical for MDV1 because it is a cell-associated virus and must maintain cap-dependent translation despite the inherent cellular stress caused by viral infection; and at the same time translate a subset of transcripts that require cap-independent translation initiation. When we first reported that the dominant isoform variant of the pp14-encoding bicistronic IE mRNA from MDV1 harbours the 5L IRES [22] we speculated on the possible strategies used by the virus to avoid the negative effect of the cap structure on the activity of the 5L IRES that controls the translation of pp14b isoform. Here, we provide evidence to support just such a strategy; whereby poly(A) tail-independent recruitment of the initiation factor PABP1 to an internal poly(A) within the 5L IRES specifically enhances translation. We propose a model that may offer mechanistic explanation as to how the 5L IRES activity is maintained and enhanced despite the competitive effect of the cap structure at the 5' end. In this model (Fig. 8), the internal poly(A) of the 5L IRES recruits PABP1 to the 5' end of the mRNA, which together with the well-known interaction between PABP1 and the poly(A) tail of the message, would lead to the circularization of the naturally occurring bicistronic dual IRES IE-mRNA. The circularization of the message may or may not necessarily contribute to the activity of the 5L IRES because even in the absence of the poly(A) tail the activity of the 5L IRES is maintained (Fig. 3C). Recent findings using cryo-electron tomography and showing that circular polyribosomes can be formed on eukaryotic mRNA without cap-structure and poly(A) tail [37], reinforce the validity of the “closed-loop” topology in the case of the naturally occurring bicistronic dual IRES IE-mRNA from MDV1, even though the mechanisms of non-covalent closure of the polyribosome rings still remain unsolved [37]. The recruitment of PABP1 to the 5L IRES via the internal poly(A) sequence may indirectly be facilitated by the action of virally encoded miRNAs that decrease the level of paip2, the inhibitor of PABP1, therefore leading to an increase in the available pool of active PABP1 (Fig. 8). Although not depicted in our model, the circularization may also be facilitated by protein-protein interactions between separate PABP1 molecules bound in the 5L IRES and the poly(A) tail; such interactions can be mediated by the proline- and glutamine-rich linker located between RRMs and the PABC domain as previously reported [38]. Additional work is needed to dissect the role of other major translation initiation factors such as eIF4G and how they may affect or not the activity of the 5L IRES. Nonetheless, this proposed model seems to be further supported by binding affinities data between the 5L IRES and the PABP1 as discussed below.


Poly(A) binding protein 1 enhances cap-independent translation initiation of neurovirulence factor from avian herpesvirus.

Tahiri-Alaoui A, Zhao Y, Sadigh Y, Popplestone J, Kgosana L, Smith LP, Nair V - PLoS ONE (2014)

Model depicting the closed loop topology for the bicistronic immediate-early transcript (IE) that encodes RLORF9 and the pp14 from MDV1.In this model, only the pp14b isoform is shown, which is under the translation control of the 5L IRES. The internal poly(A) of the 5L IRES recruits PABP1 to the 5' end of the mRNA, which may be concomitant with the recruitment of PABP1 to the poly(A) tail of the message, leading to circularization of the bicistronic dual IRES IE-mRNA. A subset of viral miRNAs down-regulate the expression level of paip2 which is a well-known inhibitor of PABP1. This down regulation of paip2 indirectly contributes to an increased level of the available pool of active PABP1 which interacts with the internal poly(A) sequence of the 5L IRES hence leading to increased IRES activity.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4263670&req=5

pone-0114466-g008: Model depicting the closed loop topology for the bicistronic immediate-early transcript (IE) that encodes RLORF9 and the pp14 from MDV1.In this model, only the pp14b isoform is shown, which is under the translation control of the 5L IRES. The internal poly(A) of the 5L IRES recruits PABP1 to the 5' end of the mRNA, which may be concomitant with the recruitment of PABP1 to the poly(A) tail of the message, leading to circularization of the bicistronic dual IRES IE-mRNA. A subset of viral miRNAs down-regulate the expression level of paip2 which is a well-known inhibitor of PABP1. This down regulation of paip2 indirectly contributes to an increased level of the available pool of active PABP1 which interacts with the internal poly(A) sequence of the 5L IRES hence leading to increased IRES activity.
Mentions: As a nuclear DNA virus, MDV1 produces capped and polyadenylated mRNAs that are indistinguishable from host mRNAs. Therefore, MDV1 has to compete with the host for resources required for mRNA translation. An elegant and detailed account of how viruses gain control of key cellular signalling pathways and subvert the host protein synthesis machinery was previously reviewed [1], [2]. The temporal gene expression of MDV1 suggests that IE and late gene expression might use different strategies of translation control so that viral protein synthesis ensues with minimal disruption to the host. This is critical for MDV1 because it is a cell-associated virus and must maintain cap-dependent translation despite the inherent cellular stress caused by viral infection; and at the same time translate a subset of transcripts that require cap-independent translation initiation. When we first reported that the dominant isoform variant of the pp14-encoding bicistronic IE mRNA from MDV1 harbours the 5L IRES [22] we speculated on the possible strategies used by the virus to avoid the negative effect of the cap structure on the activity of the 5L IRES that controls the translation of pp14b isoform. Here, we provide evidence to support just such a strategy; whereby poly(A) tail-independent recruitment of the initiation factor PABP1 to an internal poly(A) within the 5L IRES specifically enhances translation. We propose a model that may offer mechanistic explanation as to how the 5L IRES activity is maintained and enhanced despite the competitive effect of the cap structure at the 5' end. In this model (Fig. 8), the internal poly(A) of the 5L IRES recruits PABP1 to the 5' end of the mRNA, which together with the well-known interaction between PABP1 and the poly(A) tail of the message, would lead to the circularization of the naturally occurring bicistronic dual IRES IE-mRNA. The circularization of the message may or may not necessarily contribute to the activity of the 5L IRES because even in the absence of the poly(A) tail the activity of the 5L IRES is maintained (Fig. 3C). Recent findings using cryo-electron tomography and showing that circular polyribosomes can be formed on eukaryotic mRNA without cap-structure and poly(A) tail [37], reinforce the validity of the “closed-loop” topology in the case of the naturally occurring bicistronic dual IRES IE-mRNA from MDV1, even though the mechanisms of non-covalent closure of the polyribosome rings still remain unsolved [37]. The recruitment of PABP1 to the 5L IRES via the internal poly(A) sequence may indirectly be facilitated by the action of virally encoded miRNAs that decrease the level of paip2, the inhibitor of PABP1, therefore leading to an increase in the available pool of active PABP1 (Fig. 8). Although not depicted in our model, the circularization may also be facilitated by protein-protein interactions between separate PABP1 molecules bound in the 5L IRES and the poly(A) tail; such interactions can be mediated by the proline- and glutamine-rich linker located between RRMs and the PABC domain as previously reported [38]. Additional work is needed to dissect the role of other major translation initiation factors such as eIF4G and how they may affect or not the activity of the 5L IRES. Nonetheless, this proposed model seems to be further supported by binding affinities data between the 5L IRES and the PABP1 as discussed below.

Bottom Line: We report a novel viral translational control strategy involving the recruitment of PABP1 to the 5' leader internal ribosome entry site (5L IRES) of an immediate-early (IE) bicistronic mRNA that encodes the neurovirulence protein (pp14) from the avian herpesvirus Marek's disease virus serotype 1 (MDV1).We provide evidence for the interaction between an internal poly(A) sequence within the 5L IRES and PABP1 which may occur concomitantly with the recruitment of PABP1 to the poly(A) tail.RNA interference and reverse genetic mutagenesis results show that a subset of virally encoded-microRNAs (miRNAs) targets the inhibitor of PABP1, known as paip2, and therefore plays an indirect role in PABP1 recruitment strategy by increasing the available pool of active PABP1.

View Article: PubMed Central - PubMed

Affiliation: The Pirbright Institute, Ash Road, Pirbright, Woking, Surrey, United Kingdom.

ABSTRACT
Poly(A) binding protein 1 (PABP1) plays a central role in mRNA translation and stability and is a target by many viruses in diverse manners. We report a novel viral translational control strategy involving the recruitment of PABP1 to the 5' leader internal ribosome entry site (5L IRES) of an immediate-early (IE) bicistronic mRNA that encodes the neurovirulence protein (pp14) from the avian herpesvirus Marek's disease virus serotype 1 (MDV1). We provide evidence for the interaction between an internal poly(A) sequence within the 5L IRES and PABP1 which may occur concomitantly with the recruitment of PABP1 to the poly(A) tail. RNA interference and reverse genetic mutagenesis results show that a subset of virally encoded-microRNAs (miRNAs) targets the inhibitor of PABP1, known as paip2, and therefore plays an indirect role in PABP1 recruitment strategy by increasing the available pool of active PABP1. We propose a model that may offer a mechanistic explanation for the cap-independent enhancement of the activity of the 5L IRES by recruitment of a bona fide initiation protein to the 5' end of the message and that is, from the affinity binding data, still compatible with the formation of 'closed loop' structure of mRNA.

Show MeSH
Related in: MedlinePlus