Abstract
Viral RNA elements that facilitate mRNA export are useful tools for identifying cellular RNA export factors. Here we show that hepatitis B virus post-transcriptional element (PRE) is one such element, and using PRE several new cellular mRNA export factors were identified. We found that PRE drastically enhances the cytoplasmic accumulation of cDNA transcripts independent of any viral protein. Systematic deletion analysis revealed the existence of a 116 nt functional Sub-Element of PRE (SEP1). The RNP that forms on the SEP1 RNA was affinity purified, in which TREX components as well as several other proteins were identified. TREX components and the SEP1-associating protein ZC3H18 are required for SEP1-mediated mRNA export. Significantly, ZC3H18 directly binds to the SEP1 RNA, interacts with TREX and is required for stable association of TREX with the SEP1-containing mRNA. Requirements for SEP1-mediated mRNA export are similar to those for splicing-dependent mRNA export. Consistent with these similarities, several SEP1-interacting proteins, including ZC3H18, ARS2, Acinus and Brr2, are required for efficient nuclear export of polyA RNAs. Together, our data indicate that SEP1 enhances mRNA export by recruiting TREX via ZC3H18. The new mRNA export factors that we identified might be involved in cap- and splicing-dependent TREX recruitment to cellular mRNAs.
Highlights
MRNA export is a critical step in eukaryotic gene expression
When posttranscriptional element (PRE) was inserted at the 3 end, the Smad cDNA transcript that was otherwise retained in the nucleus was accumulated in the cytoplasm to the similar extent to the spliced Smad mRNA (Supplementary Figure S1)
The TREX complex, which functions in mRNA export, is recruited to spliced mRNAs dependent upon splicing and to naturally intronless mRNAs dependent upon cis-acting RNA elements that are present in these mRNAs
Summary
MRNA export is a critical step in eukaryotic gene expression It mainly occurs via specific interactions between the mRNA export receptor, known as TAP/P15, and the mRNA export adaptors, Aly and Thoc, which are components of the TREX complex (TREX) [1,2]. Except for Aly and Thoc, the human TREX contains UAP56/URH49 as well as the five other components of the six-subunit THO complex (Thoc, Thoc, Thoc, Thoc, Thoc and Tex1) [3,4]. Both formation and function of TREX are conserved from yeast, drosophila to humans. In addition to Aly, UAP56/URH49 and THO, increasing number of human TREX components have been reported, and these components play important roles in mRNA export [8,9,10]
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