RNA helicase A (RHA) is a highly conserved DEAD-box protein that activates transcription, modulates RNA splicing and binds the nuclear pore complex. The life cycle of typical mRNA involves RNA processing and translation after ribosome scanning of a relatively unstructured 5' untranslated region (UTR). The precursor RNAs of retroviruses and selected cellular genes harbor a complex 5' UTR and use a yet-to-be-identified host post-transcriptional effector to stimulate efficient translation. Here we show that RHA recognizes a structured 5'-terminal post-transcriptional control element (PCE) of a retrovirus and the JUND growth-control gene. RHA interacts with PCE RNA in the nucleus and cytoplasm, facilitates polyribosome association and is necessary for its efficient translation. Our results reveal a previously unidentified role for RHA in translation and implicate RHA as an integrative effector in the continuum of gene expression from transcription to translation.
Retroviruses rely on host RNA-binding proteins to modulate various steps in their replication. Previously several animal retroviruses were determined to mediate Dhx9/RNA helicase A (RHA) interaction with a 5′ terminal post-transcriptional control element (PCE) for efficient translation. Herein PCE reporter assays determined HTLV-1 and HIV-1 RU5 confer orientation-dependent PCE activity. The effect of Dhx9/RHA down-regulation and rescue with siRNA-resistant RHA on expression of HIV-1NL4–3 provirus determined that RHA is necessary for efficient HIV-1 RNA translation and requires ATPase-dependent helicase function. Quantitative analysis determined HIV-1 RNA steady-state and cytoplasmic accumulation were not reduced; rather the translational activity of viral RNA was reduced. Western blotting determined that RHA-deficient virions assemble with Lys-tRNA synthetase, exhibit processed reverse transcriptase and contain similar level of viral RNA, but they are poorly infectious on primary lymphocytes and HeLa cells. The results demonstrate RHA is an important host factor within the virus-producer cell and within the viral particle. The identification of RHA-dependent PCE activity in cellular junD RNA and in six of seven genera of Retroviridae suggests conservation of this translational control mechanism among vertebrates, and convergent evolution of Retroviridae to utilize this host mechanism.
RNA-templated RNA replication is essential for viral or viroid infectionAccording to the "RNA world" scenario, the appearance of RNA molecules simultaneously capable of self-replication and information storage signaled a major milestone in the evolution of life (14,24,25). In modern biology, RNA replication is central to viral or viroid infection, as well as to the regulation of cellular gene expression. Virus-encoded RNA-dependent RNA polymerases play a major role in the replication of RNA viruses (10, 23). Cellular RNA-dependent RNA polymerases generate double-stranded RNAs as triggers for RNA silencing (1). Intriguingly, the DNA-dependent cellular RNA polymerases can also transcribe at least two types of RNA templates: viroid RNAs (11,15,59) and the human hepatitis delta virus (HDV) RNA (28, 60). The replication of viroid and HDV RNAs raises the question of whether the DNA-templated transcription machinery also replicates other cellular RNAs yet to be identified. Elucidating the replication mechanisms of these infectious RNAs should help address this question of profound biological interest.Viroids are the smallest known nucleic acid-based infectious agents and self-replicating genetic units. Their "genomes" consist of single-stranded, circular RNAs ranging in size from 250 to 400 nucleotides (23). Viroids can replicate and spread throughout an infected plant, although they do not encode proteins, do not have encapsidation mechanisms, and do not require helper viruses. Furthermore, they cause devastating diseases by altering host gene expression and developmental processes (11,59). Evidently, viroid RNA genomes contain all of the sequence and structural information needed to mediate or trigger the various functions associated with infection. Potato spindle tuber viroid (PSTVd) is the type member of the family Pospiviroidae (8, 12). The PSTVd genome consists of 359 nucleotides and assumes a rod-shaped secondary structure in the native state (48) with five structural domains, as shown in Fig. 1A (26). This secondary structure, which is typical of viroids in the family Pospiviroidae, comprises many loops and bulges flanked by short Watson-Crick helices. Formation of this secondary structure is necessary for infection (62). During asymmetric rolling-circle replication of PSTVd (5), the plus circular strands serve as templates for the synthesis of concatemeric, linear minus strands, which then function as the replication intermediates for the synthesis of concatemeric, linear plus strands. These are subsequently cleaved into monomers and ligated into circular molecules (Fig. 1B). Without encoding proteins, PSTVd replicates in the nucleus of a host cell and
The stem cell protein Lin28 functions to inhibit the biogenesis of a group of miRNAs but also stimulates the expression of a subset of mRNAs at the post-transcriptional level, the underlying mechanism of which is not yet understood. Here we report the characterization of the molecular interplay between Lin28 and RNA helicase A (RHA) known to play an important role in remodeling ribonucleoprotein particles during translation. We show that reducing Lin28 expression results in decreased RHA association with polysomes while increasing Lin28 expression leads to elevated RHA association. Further, the carboxyl terminus of Lin28 is necessary for interaction with both the amino and carboxyl termini of RHA. Importantly, a carboxyl terminal deletion mutant of Lin28 that retains RNA-binding activity fails to interact with RHA and exhibits dominant-negative effects on Lin28-dependent stimulation of translation. Taken together, these results lead us to suggest that Lin28 may stimulate translation by actively recruiting RHA to polysomes.
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