The export of mature messenger RNA (mRNA) to the cytoplasm represents the culmination of the nuclear phase of the gene expression pathway (1–4
). Transport through nuclear pore complexes (NPCs) is mediated primarily by the NXF1 export factor, but this process has to be tightly coupled to earlier steps in the pathway to ensure that they have been completed successfully (1–5
). In mammalian cells, GANP (germinal-centre associated nuclear protein) integrates transcription/processing with nuclear export by facilitating the movement to nuclear pores of mRNPs generated deep in the nucleus (6
). GANP binds directly to NXF1 through its N-terminal domain and is thought to shuttle between NPCs and nuclear processing centres to which it becomes relocated when transcription is inhibited (6
). GANP is the mammalian orthologue of the yeast Sac3 protein that forms the scaffold of the TREX-2 transcription-export complex that also contains Thp1, Sem1, Sus1 and Cdc31 (7–12
) (the corresponding mammalian proteins are PCID2, DSS1, ENY2 and centrins, respectively). The way in which GANP contributes to the gene expression pathway in metazoans is fundamentally different from the way in which its partial analogue, Sac3, contributes in budding yeast. Thus, in yeast, the Sac3-based TREX-2 complex is important in localizing a subset of actively transcribing genes to NPCs (11
) to facilitate the export of their transcripts in a process referred to as ‘gene gating’ (14
). However, in metazoa, GANP appears to function primarily in chaperoning mature mRNPs as they pass from processing centres to nuclear pores (6
). Thus, in contrast to yeast, the majority of active genes in mammalian cells lie in transcription foci or ‘factories’ deep within the nucleus (15
). Indeed, examination of the dynamics of single mRNPs in human U20S cells indicates that they accumulate in a particulate pattern in the nucleoplasm similar to that seen with the transcription factories (16
). Importantly, the nuclear distribution of specific mRNPs is not influenced by the relative position of the transcription site in relation to the nuclear envelope (16
). These fundamental differences may reflect the increased nuclear size, greater incidence of introns and genome complexity of metazoans.
In budding yeast, all the components of the TREX-2 complex are required for efficient mRNA nuclear export (7
). We show here that although in human cells GANP forms an analogous TREX-2 complex, its contribution to the gene expression pathway and the integration of transcription/processing with nuclear export differs from that made by the analogous complex in yeast. Thus, although the human TREX-2 complex has a similar composition to its yeast analogue, there are differences in the way in which ENY2 is bound, and, importantly, the localization of the complex to NPCs requires the MCM3AP domain of GANP in addition to the presence of ENY2 and centrins. Furthermore, both GANP and ENY2 are associated with RNA polymerase II, and inhibition of mRNA processing redistributes GANP from NPCs into nuclear foci, suggesting that TREX-2 is associated with transcription.