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Nuclear export of human hepatitis B virus core protein and pregenomic RNA depends on the cellular NXF1-p15 machinery.

Yang CC, Huang EY, Li HC, Su PY, Shih C - PLoS ONE (2014)

Bottom Line: Cytoplasm-predominant HBc is clinically associated with severe liver inflammation.Cytoplasmic HBV pgRNA appeared to be reduced by siRNA treatment specific for the NXF1-p15 complex by quantitative RT-qPCR and Northern blot analyses.This result suggests that the pgRNA was also exported via the NXF1-p15 machinery.

View Article: PubMed Central - PubMed

Affiliation: Taiwan International Graduate Program (TIGP) in Molecular Medicine, National Yang-Ming University and Academia Sinica, Taipei, Taiwan; Institute of Biomedical Sciences, Academia Sinica, Taipei, Taiwan.

ABSTRACT
Hepatitis B virus (HBV) core protein (HBc) can shuttle between nucleus and cytoplasm. Cytoplasm-predominant HBc is clinically associated with severe liver inflammation. Previously, we found that HBc arginine-rich domain (ARD) can associate with a host factor NXF1 (TAP) by coimmunoprecipitation. It is well known that NXF1-p15 heterodimer can serve as a major export receptor of nuclear mRNA as a ribonucleoprotein complex (RNP). In the NXF1-p15 pathway, TREX (transcription/export) complex plays an important role in coupling nuclear pre-mRNA processing with mRNA export in mammalian cells. Here, we tested the hypothesis whether HBc and HBV specific RNA can be exported via the TREX and NXF1-p15 mediated pathway. We demonstrated here that HBc can physically and specifically associate with TREX components, and the NXF1-p15 export receptor by coimmunoprecipitation. Accumulation of HBc protein in the nucleus can be induced by the interference with TREX and NXF1-p15 mediated RNA export machinery. HBV transcripts encodes a non-spliced 3.5 kb pregenomic RNA (pgRNA) which can serve as a template for reverse transcription. Cytoplasmic HBV pgRNA appeared to be reduced by siRNA treatment specific for the NXF1-p15 complex by quantitative RT-qPCR and Northern blot analyses. This result suggests that the pgRNA was also exported via the NXF1-p15 machinery. We entertain the hypothesis that HBc protein can be exported as an RNP cargo via the mRNA export pathway by hijacking the TREX and NXF1-p15 complex. In our current and previous studies, HBc is not required for pgRNA accumulation in the cytoplasm. Furthermore, HBc ARD can mediate nuclear export of a chimeric protein containing HBc ARD in a pgRNA-independent manner. Taken together, it suggests that while both pgRNA and HBc protein exports are dependent on NXF1-p15, they are using the same export machinery in a manner independent of each other.

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A graphic summary of the nuclear export of HBV pgRNA and HBc.A) A cartoon illustrates a putative nuclear export pathway of HBc protein. HBV DNA genome can exist episomally as a covalently closed circular form (ccc DNA) in the nucleus, which can serve as a template for pol-II mediated RNA transcription [3]. Pre-mRNA processing and nuclear export are known to be tightly coupled by the TREX complex, which consists of ALY, BAT1, and others (other known or unknown cellular factors) [65]. HBc protein can associate with ALY, but not BAT1. Dotted line indicates that the exact sequence or molecular mechanism remains unclear. HBc protein can also associate with NXF1 and p15. It is speculated here that the NXF1-p15 export machinery can recognize a stable RNP complex between HBc protein and RNA of either viral or cellular origins. B) Nuclear export of the 3.5 kb non-spliced pgRNA is dependent on the NXF1-p15 machinery. The exact molecular mechanism of pgRNA export remains to be further investigated in the future.
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pone-0106683-g009: A graphic summary of the nuclear export of HBV pgRNA and HBc.A) A cartoon illustrates a putative nuclear export pathway of HBc protein. HBV DNA genome can exist episomally as a covalently closed circular form (ccc DNA) in the nucleus, which can serve as a template for pol-II mediated RNA transcription [3]. Pre-mRNA processing and nuclear export are known to be tightly coupled by the TREX complex, which consists of ALY, BAT1, and others (other known or unknown cellular factors) [65]. HBc protein can associate with ALY, but not BAT1. Dotted line indicates that the exact sequence or molecular mechanism remains unclear. HBc protein can also associate with NXF1 and p15. It is speculated here that the NXF1-p15 export machinery can recognize a stable RNP complex between HBc protein and RNA of either viral or cellular origins. B) Nuclear export of the 3.5 kb non-spliced pgRNA is dependent on the NXF1-p15 machinery. The exact molecular mechanism of pgRNA export remains to be further investigated in the future.

Mentions: As summarized in Fig. 9, we illustrated putative nuclear export pathways of HBc protein (Fig. 9A) and pgRNA (Fig. 9B), respectively. We hypothesized here that HBc protein can get exported by usurping the cellular export machinery of spliced RNAs, including TREX and NXF1-p15 complex (Fig. 9A). Along the way to its nuclear export, HBc probably can form a messenger ribonucleoprotein (mRNP) complex with several RNA-binding adaptor proteins. These mRNP cargos can then target to nuclear pore complex (NPC) of the nuclear envelope before translocation into the cytoplasm. In contrast to the export of HBc protein, we have no evidence that the export of HBV pgRNA requires TREX, albeit it is dependent on NXF1-p15 (Fig. 9B).


Nuclear export of human hepatitis B virus core protein and pregenomic RNA depends on the cellular NXF1-p15 machinery.

Yang CC, Huang EY, Li HC, Su PY, Shih C - PLoS ONE (2014)

A graphic summary of the nuclear export of HBV pgRNA and HBc.A) A cartoon illustrates a putative nuclear export pathway of HBc protein. HBV DNA genome can exist episomally as a covalently closed circular form (ccc DNA) in the nucleus, which can serve as a template for pol-II mediated RNA transcription [3]. Pre-mRNA processing and nuclear export are known to be tightly coupled by the TREX complex, which consists of ALY, BAT1, and others (other known or unknown cellular factors) [65]. HBc protein can associate with ALY, but not BAT1. Dotted line indicates that the exact sequence or molecular mechanism remains unclear. HBc protein can also associate with NXF1 and p15. It is speculated here that the NXF1-p15 export machinery can recognize a stable RNP complex between HBc protein and RNA of either viral or cellular origins. B) Nuclear export of the 3.5 kb non-spliced pgRNA is dependent on the NXF1-p15 machinery. The exact molecular mechanism of pgRNA export remains to be further investigated in the future.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4215830&req=5

pone-0106683-g009: A graphic summary of the nuclear export of HBV pgRNA and HBc.A) A cartoon illustrates a putative nuclear export pathway of HBc protein. HBV DNA genome can exist episomally as a covalently closed circular form (ccc DNA) in the nucleus, which can serve as a template for pol-II mediated RNA transcription [3]. Pre-mRNA processing and nuclear export are known to be tightly coupled by the TREX complex, which consists of ALY, BAT1, and others (other known or unknown cellular factors) [65]. HBc protein can associate with ALY, but not BAT1. Dotted line indicates that the exact sequence or molecular mechanism remains unclear. HBc protein can also associate with NXF1 and p15. It is speculated here that the NXF1-p15 export machinery can recognize a stable RNP complex between HBc protein and RNA of either viral or cellular origins. B) Nuclear export of the 3.5 kb non-spliced pgRNA is dependent on the NXF1-p15 machinery. The exact molecular mechanism of pgRNA export remains to be further investigated in the future.
Mentions: As summarized in Fig. 9, we illustrated putative nuclear export pathways of HBc protein (Fig. 9A) and pgRNA (Fig. 9B), respectively. We hypothesized here that HBc protein can get exported by usurping the cellular export machinery of spliced RNAs, including TREX and NXF1-p15 complex (Fig. 9A). Along the way to its nuclear export, HBc probably can form a messenger ribonucleoprotein (mRNP) complex with several RNA-binding adaptor proteins. These mRNP cargos can then target to nuclear pore complex (NPC) of the nuclear envelope before translocation into the cytoplasm. In contrast to the export of HBc protein, we have no evidence that the export of HBV pgRNA requires TREX, albeit it is dependent on NXF1-p15 (Fig. 9B).

Bottom Line: Cytoplasm-predominant HBc is clinically associated with severe liver inflammation.Cytoplasmic HBV pgRNA appeared to be reduced by siRNA treatment specific for the NXF1-p15 complex by quantitative RT-qPCR and Northern blot analyses.This result suggests that the pgRNA was also exported via the NXF1-p15 machinery.

View Article: PubMed Central - PubMed

Affiliation: Taiwan International Graduate Program (TIGP) in Molecular Medicine, National Yang-Ming University and Academia Sinica, Taipei, Taiwan; Institute of Biomedical Sciences, Academia Sinica, Taipei, Taiwan.

ABSTRACT
Hepatitis B virus (HBV) core protein (HBc) can shuttle between nucleus and cytoplasm. Cytoplasm-predominant HBc is clinically associated with severe liver inflammation. Previously, we found that HBc arginine-rich domain (ARD) can associate with a host factor NXF1 (TAP) by coimmunoprecipitation. It is well known that NXF1-p15 heterodimer can serve as a major export receptor of nuclear mRNA as a ribonucleoprotein complex (RNP). In the NXF1-p15 pathway, TREX (transcription/export) complex plays an important role in coupling nuclear pre-mRNA processing with mRNA export in mammalian cells. Here, we tested the hypothesis whether HBc and HBV specific RNA can be exported via the TREX and NXF1-p15 mediated pathway. We demonstrated here that HBc can physically and specifically associate with TREX components, and the NXF1-p15 export receptor by coimmunoprecipitation. Accumulation of HBc protein in the nucleus can be induced by the interference with TREX and NXF1-p15 mediated RNA export machinery. HBV transcripts encodes a non-spliced 3.5 kb pregenomic RNA (pgRNA) which can serve as a template for reverse transcription. Cytoplasmic HBV pgRNA appeared to be reduced by siRNA treatment specific for the NXF1-p15 complex by quantitative RT-qPCR and Northern blot analyses. This result suggests that the pgRNA was also exported via the NXF1-p15 machinery. We entertain the hypothesis that HBc protein can be exported as an RNP cargo via the mRNA export pathway by hijacking the TREX and NXF1-p15 complex. In our current and previous studies, HBc is not required for pgRNA accumulation in the cytoplasm. Furthermore, HBc ARD can mediate nuclear export of a chimeric protein containing HBc ARD in a pgRNA-independent manner. Taken together, it suggests that while both pgRNA and HBc protein exports are dependent on NXF1-p15, they are using the same export machinery in a manner independent of each other.

Show MeSH
Related in: MedlinePlus