3.A.22 The Transcription-coupled TREX/TAP Nuclear mRNA Export Complex (TREX) Family
Although the vast majority of pre-mRNAs in higher eukaryotes contain introns, 5% of human protein-coding genes do not (Shabalina et al., 2010). These intronless mRNAs encode proteins of critical importance, including the histones, the c-Jun proto-oncoprotein, and the antiviral IFN proteins. In addition, many viral genes lack introns. Studies over the past several years have revealed that splicing is not only essential for removing introns but also plays roles in other steps in gene expression because of functional coupling among the different steps. For example, splicing is coupled to RNA polymerase II (RNAP II) transcription, 3′ end formation, mRNA stability, mRNA export, translation, and cytoplasmic localization of mRNA (Cramer et al., 2001). mRNAs generated by splicing are both more stable and more efficiently exported to the cytoplasm than their cDNA transcript counterparts (Reed and Hurt, 2002). A polyadenylation signal, TREX mRNA export components, and the mRNA export receptor TAP are required for accumulation of the naturally intronless mRNAs in the cytoplasm. Lei et al. (Lei et al., 2011) concluded that naturally intronless mRNAs contain specific sequences that result in efficient packaging into the TREX mRNA export complex supplant the splicing requirement for efficient mRNA export.
Some of the yeast TREX export complex components are distantly related to the vertebrate Nuclear mRNA Exporter (mRNA-E) family (3.A.18), and some are found in the Nuclear Pore Complex (NPC) family (1.I.1).
The reaction catalyzed by the TRER export system is:
mRNA (nucleus) → mRNA (cytoplasm)