Murine Leukemia Virus Uses TREX Components for Efficient Nuclear Export of Unspliced Viral Transcripts

Jason Tonne,Yasuhiro Ikeda,Toshie Sakuma

doi:10.3390/v6031135

Abstract

Previously we reported that nuclear export of both unspliced and spliced murine leukemia virus (MLV) transcripts depends on the nuclear export factor (NXF1) pathway. Although the mRNA export complex TREX, which contains Aly/REF, UAP56, and the THO complex, is involved in the NXF1-mediated nuclear export of cellular mRNAs, its contribution to the export of MLV mRNA transcripts remains poorly understood. Here, we studied the involvement of TREX components in the export of MLV transcripts. Depletion of UAP56, but not Aly/REF, reduced the level of both unspliced and spliced viral transcripts in the cytoplasm. Interestingly, depletion of THO components, including THOC5 and THOC7, affected only unspliced viral transcripts in the cytoplasm. Moreover, the RNA immunoprecipitation assay showed that only the unspliced viral transcript interacted with THOC5. These results imply that MLV requires UAP56, THOC5 and THOC7, in addition to NXF1, for nuclear export of viral transcripts. Given that naturally intronless mRNAs, but not bulk mRNAs, require THOC5 for nuclear export, it is plausible that THOC5 plays a key role in the export of unspliced MLV transcripts.

Highlights

Nucleocytoplasmic transport of mRNA is regulated by RNA-binding proteins
We have previously shown that nuclear export factor 1 (NXF1) is involved in murine leukemia virus (MLV) mRNA export [14]
To assess the involvement of transcription export (TREX) components in the nuclear export of MLV transcripts, we first examined whether disruption of major components of TREX, UAP56 and Aly/REF, affects the expression of MLV proteins (Figure 1)

Summary

Introduction

Nucleocytoplasmic transport of mRNA is regulated by RNA-binding proteins. Two major classes of transport receptors are known to mediate viral RNA export: nuclear export factor 1 (NXF1) and karyopherins, known as importins/exportins [1]. Most retroviruses, including human immunodeficiency virus type 1 (HIV-1), mouse mammary tumor virus, and human T cell leukemia virus encode viral proteins Rev, Rem and Rex, respectively These viral proteins interact within a cis-element in the viral genome, and promote nuclear export of intron-containing viral RNAs through a karyopherin, CRM1 (chromosome region maintenance 1), known as XPO1 (exportin1) [3,4,5,6,7,8,9,10,11]. A TREX component of RNA helicase UAP56 is first recruited to the mRNA along with the adaptor protein Aly/REF [31,32,33]. THO complex by siRNAs [38], we determined that THOC5 and THOC7 are required for efficient nuclear export of unspliced viral RNAs

Involvement of UAP56 in the Nuclear Export of MLV Transcripts

Cell Lines and Transfection

Northern Blotting

Western Blot

Conclusions