友廣 拓生

The neuronal ELAV protein HuD is known to promote neuronal differentiation and stimulate cap-dependent translation, but the underlying mechanism remains incompletely understood. Here, we identify a functional domain within the HuD linker that governs interaction with the nuclear export factor TAP/NXF1 and stimulates translation. Using a series of deletion mutants, we show that TAP/NXF1 binding is functionally separable from cytoplasmic localization, and that loss of this interaction correlates with impaired translation and neurite outgrowth. These findings reveal a critical subdomain within HuD that recruits TAP/NXF1 to promote translation and neuronal differentiation, establishing a mechanistic link between RNA export factors and ELAV-mediated translational control.

Transducer of ErbB2 (TOB) proteins have been shown to promote mRNA decay through interactions with the CCR4-NOT complex and poly(A)-binding protein (PABP). While their role in deadenylation-mediated mRNA degradation is well established, their potential function in translational control remains to be elucidated. Here, we employed an in vitro translation system combined with an RNA tethering strategy to examine the function of TOB1 and TOB2 in translation. Our results demonstrate that TOB1 and TOB2 act as repressors of translation initiation, independent of deadenylation. Notably, this translational repression selectively targets eIF4A-dependent translation, while translation driven by eIF4A-independent IRES elements remains unaffected. While the interaction between TOB proteins and PABP appears to be dispensable, as disruption of this interaction only partially reduces translational repression, the knockdown of CNOT1, the scaffold of the CCR4-NOT complex, substantially relieves this repression, highlighting its indispensable role in the mechanism. Collectively, our findings uncover a previously unrecognized function of TOB proteins as direct repressors of translation initiation, independent of mRNA decay, and highlight a specific reliance on eIF4A activity and CCR4-NOT complex integrity.

The migration of gonadal distal tip cells (DTCs) in Caenorhabditis elegans serves as an excellent model for studying the migration of epithelial tubes during organogenesis. Mutations in the mig-17/ADAMTS gene cause misdirected DTC migration during gonad formation, resulting in deformed gonad arms. An amino acid substitution in RPL-20, the ortholog of mammalian RPL18a/eL20, a component of the 60 S ribosomal large subunit, exhibited a slow-growth phenotype and strongly suppressed the mig-17 gonadal defects. Slow-growing mutations clk-1 and clk-2 also suppressed mig-17. Intestine-specific overexpression of mutant RPL-20 protein resulted in a slow-growth phenotype and suppressed the mig-17 gonadal defects, but these effects were much weaker when wild-type RPL-20 was overexpressed, suggesting that the mutant RPL-20 protein acquired a novel function. Analysis of ribosome profiles revealed reduced biogenesis of the 60 S subunit, leading to a reduction of 80 S ribosomes in the rpl-20 mutant. These results suggest that DTC migration defects in mig-17/ADAMTS mutants can be partly suppressed by growth retardation caused by the rpl-20 mutation. While defective ribosome biogenesis may contribute to the observed growth retardation, further investigation is needed to clarify the molecular basis of this phenomenon.

Stress granules (SGs) are dense aggregates of RNA and proteins that form in response to various cellular stresses. Virus-induced SGs, known as antiviral SGs (avSGs), play a crucial role in regulating retinoic acid-inducible gene I-like receptors (RLRs)-mediated antiviral innate immunity. However, the regulation of avSG formation remains not fully understood. In this study, we demonstrate that TAR-RNA binding protein (TRBP), an RNA silencing regulator, negatively regulates type I interferon (IFN) expression by inhibiting avSG formation in response to RNA virus infection. Overexpression of TRBP inhibits both IFN-β promoter activity and avSG formation following viral infection or the viral RNA mimic, polyinosinic-polycytidylic acid transfection. TRBP knockout cells exhibit enhanced phosphorylation and activation of IFN regulatory factor-3 (IRF-3) and increased IFN-β mRNA expression compared to wild-type cells. Additionally, depletion of G3BP1 and G3BP2, which are essential for SG formation, abolishes the inhibitory effect of TRBP on IRF-3 phosphorylation. Mechanistically, TRBP physically interacts with double-stranded RNA (dsRNA)-dependent protein kinase R (PKR), a key kinase involved in avSG formation, via its dsRNA-binding domains, and inhibits PKR activation. In summary, our findings reveal a novel function for TRBP as a negative regulator of RLR-mediated signaling through PKR-dependent inhibition of avSG formation.

The RNA-binding protein HuD/ELAVL4 is essential for neuronal development and synaptic plasticity by governing various post-transcriptional processes of target mRNAs, including stability, translation, and localization. We previously showed that the linker region and poly(A)-binding domain of HuD play a pivotal role in promoting translation and inducing neurite outgrowth. In addition, we found that HuD interacts exclusively with the active form of Akt1, through the linker region. Although this interaction is essential for neurite outgrowth, HuD is not a substrate for Akt1, raising questions about the dynamics between HuD-mediated translational stimulation and its association with active Akt1. Here, we demonstrate that active Akt1 interacts with the cap-binding complex via HuD. We identify key amino acids in linker region of HuD responsible for Akt1 interaction, leading to the generation of two point-mutated HuD variants: one that is incapable of binding to Akt1 and another that can interact with Akt1 regardless of its phosphorylation status. In vitro translation assays using these mutants reveal that HuD-mediated translation stimulation is independent of its binding to Akt1. In addition, it is evident that the interaction between HuD and active Akt1 is essential for HuD-induced neurite outgrowth, whereas a HuD mutant capable of binding to any form of Akt1 leads to aberrant neurite development. Collectively, our results revisit the understanding of the HuD-Akt1 interaction in translation and suggest that this interaction contributes to HuD-mediated neurite outgrowth via a unique molecular mechanism distinct from translation regulation.