An intron with a constitutive transport element is retained in a Tap messenger RNA.
Alternative splicing is a key factor contributing to genetic diversity and evolution. Intron retention, one form of alternative splicing, is common in plants but rare in higher eukaryotes, because messenger RNAs with retained introns are subject to cellular restriction at the level of cytoplasmic export and expression. Often, retention of internal introns restricts the export of these mRNAs and makes them the targets for degradation by the cellular nonsense-mediated decay machinery if they contain premature stop codons. In fact, many of the database entries for complementary DNAs with retained introns represent them as artefacts that would not affect the proteome. Retroviruses are important model systems in studies of regulation of RNAs with retained introns, because their genomic and mRNAs contain one or more unspliced introns. For example, Mason-Pfizer monkey virus overcomes cellular restrictions by using a cis-acting RNA element known as the constitutive transport element (CTE). The CTE interacts directly with the Tap protein (also known as nuclear RNA export factor 1, encoded by NXF1), which is thought to be a principal export receptor for cellular mRNA, leading to the hypothesis that cellular mRNAs with retained introns use cellular CTE equivalents to overcome restrictions to their expression. Here we show that the Tap gene contains a functional CTE in its alternatively spliced intron 10. Tap mRNA containing this intron is exported to the cytoplasm and is present in polyribosomes. A small Tap protein is encoded by this mRNA and can be detected in human and monkey cells. Our results indicate that Tap regulates expression of its own intron-containing RNA through a CTE-mediated mechanism. Thus, CTEs are likely to be important elements that facilitate efficient expression of mammalian mRNAs with retained introns.
Li
Ying
Y
Myles H. Thaler Center for AIDS & Human Retrovirus Research and Department of Microbiology, University of Virginia, Charlottesville, Virginia 22908, USA.
Bor
Yeou-Cherng
YC
Misawa
Yukiko
Y
Xue
Yuming
Y
Rekosh
David
D
Hammarskjöld
Marie-Louise
ML
D017398:Alternative Splicing / Q000235:genetics*; D020029:Base Pairing; D001483:Base Sequence; D002460:Cell Line; D006801:Humans; D007438:Introns / Q000235:genetics*; D008969:Molecular Sequence Data; D029543:Nucleocytoplasmic Transport Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics* / Q000378:metabolism; D016601:RNA-Binding Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D038621:Regulatory Sequences, Ribonucleic Acid / Q000235:genetics*; D014162:Transfection
C122111:NXF1 protein, human; D029543:Nucleocytoplasmic Transport Proteins; D012333:RNA, Messenger; D016601:RNA-Binding Proteins; D038621:Regulatory Sequences, Ribonucleic Acid