Investigating mRNA Sequence Features that Regulate Nuclear Export

In eukaryotes, the nucleus divides the cell into two compartments: the nucleoplasm, where RNA is synthesized, processed and packaged, and the cytoplasm, where mature mRNA is translated into proteins. The mechanisms that determine whether an mRNA should be exported from the nucleus to the cytoplasm are poorly understood. The best established model postulated that mRNAs need to be spliced or to contain nuclear export-promoting sequences so as to be efficiently transported to the cytoplasm. However, this model suffered several problems since it was known that many intronless mRNAs are efficiently exported to the cytoplasm independently of splicing and they do not contain any known nuclear export-promoting sequences. In this thesis, I sought to dissect the sequence features within a transcript that determine its cellular localization. I discovered that intronless β-Globin (βG) mRNA contains a nuclear retention element that inhibits its transport to the cytoplasm. This nuclear retention element can be overcome when βG mRNA is either spliced or its length extended. The mRNA nuclear export factor UAP56 binds to several intronless mRNAs including βG mRNA independently of splicing. I also discovered that UAP56 is required for the egress of intronless mRNAs from nuclear speckles. My results suggest that most mRNAs are exported to the cytoplasm independently of splicing unless they contain a nuclear retention element.Ph.D