|dc.description.abstracteng||The DEAD-box RNA-helicase Dbp5/Rat8 is well-known for its function in nuclear mRNA export. By using its Gle1 and IP6-stimulated ATPase activity, Dbp5 displaces export factors such as Mex67 from the mRNAs at the cytoplasmic sides of the nuclear pore complexes. This Dbp5-mediated remodeling of the emerging mRNPs is anticipated to enable their directional transport from the nucleus into the cytoplasm. The present study shows that Dbp5 is also required for the nuclear export of other large ribonucleoprotein complexes, the pre-ribosomal subunits. GFP-microscopy and fluorescence in situ hybridization experiments in Saccharomyces cerevisiae reveal that both, pre-40S and pre-60S particles accumulate in the nuclei of different dbp5 mutants. Like in mRNA export, Dbp5 acts also in ribosomal transport at the cytoplasmic sides of the nuclear pore complexes, where it shortly contacts the emerging ribosomal particles. However, Dbp5 utilizes for the export of pre-ribosomal subunits a mechanism that is distinct from its role in mRNA transport. Dbp5 does not displace the export receptor Mex67 from the pre-ribosomal subunits and does not require its Gle1-stimulated ATPase cycle for ribosomal transport. Thus, the ATPase-dependent remodeling activity of Dbp5 in general is dispensable for the nuclear export of pre-ribosomal subunits. These findings uncover differences in the transport mechanisms of Dbp5 for different large ribonucleoprotein particles.
In addition to the roles in nuclear export events, Dbp5 and its co-factor Gle1 are also involved in efficient translation termination in the cytoplasm. Translation termination takes place upon arrival of the translating ribosomes at the stop codon and requires the canonical release factors eRF1 and eRF3 to mediate stop codon recognition and peptide release. Furthermore, the ATP-binding cassette protein Rli1 was described as an additional termination factor. However, the chronological order of all factors acting during translation termination is still unknown. Co-immunoprecipitation and sucrose-density gradient studies reveal a novel interaction between Dbp5 and Rli1 during translation termination and an early entry of Dbp5 into the termination complex that might depend on eRF1. Dbp5 is recruited to the ribosomes after Rli1, which already associates with ribosomes prior to translation termination. Thus, a ribosome-mediated contact between Rli1 and Dbp5 is possible either before or during the eRF1-mediated stop codon recognition. These findings present important insights into the temporal coordination of all termination factors during translation termination.||de