This competitive renewal stems from our finding that the nonsense-mediated mRNA decay (NMD) factor UPF1 has multiple functions in mammalian cells. Our result using a stringent yeast-two-hybrid assay that human UPF1 interacts with human Staufen (STAU)1 led to our discovering a pathway related to NMD that we named STAU1-mediated mRNA decay (SMD). We have demonstrated that STAU1 recruits UPF1 to the 3'- untranslated region (3'-UTR) of particular mRNAs so as to elicit mRNA decay when translation terminates normally. Unlike NMD, which functions primarily as a quality control mechanism to eliminate mRNAs that prematurely terminate translation during a pioneer round of translation, SMD appears to conditionally down- regulate the expression of genes whose mRNAs bind STAU1 within their 3'-UTRs during both pioneer and subsequent rounds of translation.
AIM 1 will continue to examine the mechanism of SMD by characterizing the factors involved, the role for STAU1 arginine methylation, the cis-acting mRNA sequences that bind STAU1, and the proteins other than STAU1 that bind SMD targets. In related studies, we have found that STAU1 can down-regulate the expression of particular genes in a way that is insensitive to UPF1 siRNA and, thus, apparently distinct from SMD. To follow up these unpublished results, AIM 2 will define the one or more STAU1-mediated pathways of post-transcriptional gene regulation that are apparently distinct from SMD. We will focus on pathways that affect mRNA metabolism, which is our forti. We have additionally discovered that hyper-phosphorylated UPF1 functions during NMD to repress translation by binding to eIF3 in a way that prevents joining of the 60S ribosomal subunit to the 40S ribosomal subunit that constitutes the preinitiation complex at the 5'-end of an NMD target.
AIM 3 will further characterize the interaction of hyper- phosphorylated UPF1 with eIF3 as a means to better understand the mechanism of translational repression. The proposed experiments logically extend our long-time studies of RNA metabolism in mammalian cells and are expected to lend important insights into aspects of SMD, one or more new STAU1-dependent pathways of post-transcriptional gene control, and translational repression during NMD.
Proper control of human gene expression requires the two mRNA decay pathways that are at the heart of this competitive renewal. Both pathways were uncovered by this P.I. Staufen1- mediated mRNA decay serves to conditionally down-regulate genes encoding mRNAs that bind Staufen1 in their 3'-untranslated regions. Nonsense-mediated mRNA decay (NMD) eliminates the expression of genes encoding mRNAs that harbor premature termination codons. By so doing, NMD protects many heterozygous carriers of disease-associated alleles from manifesting a dominantly inherited form of the disease. NMD also contributes to proper gene expression in many ways. As an example, NMD provides a means by which many RNA binding proteins negatively regulate the genes from which they derived so that an appropriate level of gene expression can be achieved.
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