Functions of hUPF3a and hUPF3b in nonsense-mediated mRNA decay and translation

Department for Pediatric Oncology, Hematology and Immunology, University of Heidelberg, Germany.
RNA (Impact Factor: 4.94). 07/2006; 12(6):1015-22. DOI: 10.1261/rna.12506
Source: PubMed


The exon-junction complex (EJC) components hUpf3a and hUpf3b serve a dual function: They promote nonsense-mediated mRNA decay (NMD), and they also regulate translation efficiency. Whether these two functions are interdependent or independent of each other is unknown. We characterized the function of the hUpf3 proteins in a lambdaN/boxB-based tethering system. Despite the high degree of sequence similarity between hUpf3b and hUpf3a, hUpf3a is much less active than hUpf3b to induce NMD and to stimulate translation. We show that induction of NMD by hUpf3 proteins requires interaction with Y14, Magoh, BTZ, and eIF4AIII. The protein region that mediates this interaction and discriminates between hUpf3a and hUpf3b in NMD function is located in the C-terminal domain and fully contained within a small sequence that is highly conserved in Upf3b but not Upf3a proteins. Stimulation of translation is independent of this interaction and is determined by other regions of the hUpf3 protein, indicating the presence of different downstream pathways of hUpf3 proteins either in NMD or in translation.

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    • "When UPF3B is depleted, this allows more UPF3A to bind to UPF2, and thus UPF3A is stabilized. While UPF3A is a weak NMD factor, at least in vitro [50], Chan et al. found it is able to at least partially functionally compensate for UPF3B when the latter factor is depleted. This cross-regulatory mechanism has the potential to influence human disease (Fig. 2B), as UPF3A protein levels are also elevated in intellectually disabled patients with germline silencing mutations in the UPF3B gene [10] [51]. "
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    ABSTRACT: Nonsense-mediated mRNA decay (NMD) is an mRNA quality control mechanism that destabilizes aberrant mRNAs harboring premature termination (nonsense) codons (PTCs). Recent studies have shown that NMD also targets mRNAs transcribed from a large subset of wild-type genes. This raises the possibility that NMD itself is under regulatory control. Indeed, several recent studies have shown that NMD activity is modulated in specific cell types and that key components of the NMD pathway are regulated by several pathways, including microRNA circuits and NMD itself. Cellular stress also modulates the magnitude of NMD by mechanisms that are beginning to be understood. Here, we review the evidence that NMD is regulated and discuss the physiological role for this regulation. We propose that the efficiency of NMD is altered in some cellular contexts to regulate normal biological events. In disease states-such as in cancer-NMD is disturbed by intrinsic and extrinsic factors, resulting in altered levels of crucial NMD-targeted mRNAs that lead to downstream pathological consequences.
    Biochimica et Biophysica Acta 03/2013; 1829(6-7). DOI:10.1016/j.bbagrm.2013.03.002 · 4.66 Impact Factor
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    • "Both genes have redundant function and link Upf2 and the EJC (Kunz et al. 2006; Chamieh et al. 2008; Melero et al. 2012). The Upf3a splice variant, Upf3aS, does not bind Upf2 but preferentially binds phospho- Upf1 and the SMG-5:SMG-7 complex (Ohnishi et al. 2003; Kunz et al. 2006). A significant role for Upf3 is not known in other organisms. "
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    ABSTRACT: SMG-1, a member of the PIKK (phosphoinositide 3-kinase-related kinase) family, plays a critical role in the mRNA quality control system known as nonsense-mediated mRNA decay (NMD). NMD protects cells from the accumulation of aberrant mRNAs with premature termination codons (PTCs) which encode nonfunctional or potentially harmful truncated proteins. SMG-1 directly phosphorylates Upf1 helicase, another key component of NMD, upon recognition of PTC on postspliced mRNA during the initial round of translation. Phosphorylated-Upf1 recruits the SMG-5/SMG-7 complex to induce ribosome dissociation and decapping-mediated decay. Phospho-Upf1 also recruits the SMG-6 endonuclease which might be involved in endo-cleavage. Upf1 ATPase/helicase activities are likely required for the activation of other mRNA decay enzymes and the mRNA-protein complex dissociation to complete NMD. At present, a variety of tools are available that can specifically suppress NMD, and it has become possible to examine the contribution of NMD in a variety of physiological and pathological conditions.
    Genes to Cells 01/2013; 18(3). DOI:10.1111/gtc.12033 · 2.81 Impact Factor
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    • "In mammalian cells, NMD involves 4 main factors: UPF1, UPF2, UPF3 (also called UPF3a) and UPF3X (also called UPF3b). The actual role of UPF proteins remains unclear and certain UPFs are not required for all NMD reactions [15-17]. UPF proteins are recruited to the mRNP in a sequential manner: UPF3 or UPF3X arriving first, then UPF2 and finally UPF1. "
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    ABSTRACT: Background Nonsense mutations are at the origin of many cancers and inherited genetic diseases. The consequence of nonsense mutations is often the absence of mutant gene expression due to the activation of an mRNA surveillance mechanism called nonsense-mediated mRNA decay (NMD). Strategies to rescue the expression of nonsense-containing mRNAs have been developed such as NMD inhibition or nonsense mutation readthrough. Methods Using a dedicated screening system, we sought molecules capable to block NMD. Additionally, 3 cell lines derived from patient cells and harboring a nonsense mutation were used to study the effect of the selected molecule on the level of nonsense-containing mRNAs and the synthesis of proteins from these mutant mRNAs. Results We demonstrate here that amlexanox, a drug used for decades, not only induces an increase in nonsense-containing mRNAs amount in treated cells, but also leads to the synthesis of the full-length protein in an efficient manner. We also demonstrated that these full length proteins are functional. Conclusions As a result of this dual activity, amlexanox may be useful as a therapeutic approach for diseases caused by nonsense mutations.
    Orphanet Journal of Rare Diseases 08/2012; 7(1):58. DOI:10.1186/1750-1172-7-58 · 3.36 Impact Factor
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