Nuclear Export of Pre-Ribosomal Subunits Requires Dbp5, but Not as an RNA-Helicase as for mRNA Export
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Bettina Neumann1, Haijia Wu1, Alexandra Hackmann1
1Abteilung für Molekulare Genetik, Institut für Mikrobiologie und Genetik, Göttinger Zentrum für Molekulare Biowissenschaften (GZMB), Georg-August Universität Göttingen, Germany.
Abstract:
The DEAD-box RNA-helicase Dbp5/Rat8 is known for its function in nuclear mRNA export, where it displaces the export receptor Mex67 from the mRNA at the cytoplasmic side of the nuclear pore complex (NPC). Here we show that Dbp5 is also required for the nuclear export of both pre-ribosomal subunits. Yeast temperature-sensitive dbp5 mutants accumulate both ribosomal particles in their nuclei. Furthermore, Dbp5 genetically and physically interacts with known ribosomal transport factors such as Nmd3. Similar to mRNA export we show that also for ribosomal transport Dbp5 is required at the cytoplasmic side of the NPC. However, unlike its role in mRNA export, Dbp5 does not seem to undergo its ATPase cycle for this function, as ATPase-deficient dbp5 mutants that selectively inhibit mRNA export do not affect ribosomal transport. Furthermore, mutants of GLE1, the ATPase stimulating factor of Dbp5, show no major ribosomal export defects. Consequently, while Dbp5 uses its ATPase cycle to displace the export receptor Mex67 from the translocated mRNAs, Mex67 remains bound to ribosomal subunits upon transit to the cytoplasm, where it is detectable on translating ribosomes. Therefore, we propose a model, in which Dbp5 supports ribosomal transport by capturing ribosomal subunits upon their cytoplasmic appearance at the NPC, possibly by binding export factors such as Mex67. Thus, our findings reveal that although different ribonucleoparticles, mRNAs and pre-ribosomal subunits, use shared export factors, they utilize different transport mechanisms.
Insights
The DEAD-box RNA-helicase Dbp5 (also known as Rat8) is crucial for both mRNA and pre-ribosomal subunit nuclear export, utilizing distinct mechanisms at the nuclear pore complex.
Area of Science:
- Molecular Biology
- Cell Biology
- Biochemistry
Background:
- The DEAD-box RNA helicase Dbp5/Rat8 is established to facilitate nuclear mRNA export by releasing the Mex67 export receptor from mRNA at the cytoplasmic face of the nuclear pore complex (NPC).
- Ribonucleoprotein (RNP) export from the nucleus is essential for gene expression and cellular function, involving specific export pathways and factors.
Purpose of the Study:
- To investigate the role of Dbp5/Rat8 in the nuclear export of pre-ribosomal subunits.
- To elucidate the mechanism by which Dbp5/Rat8 mediates ribosomal subunit export and compare it to its role in mRNA export.
Main Methods:
- Utilized yeast temperature-sensitive mutants of Dbp5/Rat8.
- Performed genetic interaction studies with known ribosomal transport factors like Nmd3.
- Investigated Dbp5/Rat8 function at the cytoplasmic side of the NPC.
- Assessed the role of Dbp5/Rat8's ATPase activity and its interaction with GLE1.
Main Results:
- Yeast dbp5 mutants exhibited nuclear accumulation of pre-ribosomal subunits, indicating a defect in their export.
- Dbp5 interacts genetically and physically with ribosomal transport factors, including Nmd3.
- Unlike mRNA export, Dbp5's ATPase-independent function appears critical for ribosomal subunit export.
- ATPase-deficient dbp5 mutants and GLE1 mutants showed no significant defects in ribosomal export, suggesting a distinct mechanism.
Conclusions:
- Dbp5/Rat8 plays a dual role in nuclear export, mediating both mRNA and pre-ribosomal subunit transport via different mechanisms.
- While Dbp5 uses its ATPase activity to displace Mex67 from mRNA, it likely binds Mex67 without ATP hydrolysis to facilitate ribosomal subunit export.
- This study reveals that shared export factors like Mex67 can be utilized differently by distinct RNPs (mRNAs and ribosomal subunits) during nuclear export.
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