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Molecular and Cellular Biology, October 2001, p. 6960-6971, Vol. 21, No. 20
0270-7306/01/$04.00+0   DOI: 10.1128/MCB.21.20.6960-6971.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

Versatile Role for hnRNP D Isoforms in the Differential Regulation of Cytoplasmic mRNA Turnover

Nianhua Xu, Chyi-Ying A. Chen, and Ann-Bin Shyu*

Department of Biochemistry and Molecular Biology, The University of Texas Houston Medical School, Houston, Texas 77030

Received 10 May 2001/Returned for modification 21 June 2001/Accepted 16 July 2001

An important emerging theme is that heterogeneous nuclear ribonucleoproteins (hnRNPs) not only function in the nucleus but also control the fates of mRNAs in the cytoplasm. Here, we show that hnRNP D plays a versatile role in cytoplasmic mRNA turnover by functioning as a negative regulator in an isoform-specific and cell-type-dependent manner. We found that hnRNP D discriminates among the three classes of AU-rich elements (AREs), most effectively blocking rapid decay directed by class II AREs found in mRNAs encoding cytokines. Our experiments identified the overlapping AUUUA motifs, one critical characteristic of class II AREs, to be the key feature recognized in vivo by hnRNP D for its negative effect on ARE-mediated mRNA decay. The four hnRNP D isoforms, while differing in their ability to block decay of ARE-containing mRNAs, all potently inhibited mRNA decay directed by another mRNA cis element that shares no sequence similarity with AREs, the purine-rich c-fos protein-coding region determinant of instability. Further experiments indicated that different mechanisms underlie the inhibitory effect of hnRNP D on the two distinct mRNA decay pathways. Our study identifies a potential mechanism by which cytoplasmic mRNA turnover can be differentially and selectively regulated by hnRNP D isoforms in mammalian cells. Our results support the notion that hnRNP D serves as a key factor broadly involved in general mRNA decay.

* Corresponding author. Mailing address: Department of Biochemistry and Molecular Biology, The University of Texas Houston Medical School, Houston, TX 77030. Phone: (713) 500-6068. Fax: (713) 500-0652. E-mail: Ann-Bin.Shyu{at}uth.tmc.edu.

Molecular and Cellular Biology, October 2001, p. 6960-6971, Vol. 21, No. 20
0270-7306/01/$04.00+0   DOI: 10.1128/MCB.21.20.6960-6971.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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