Nuclear dynamics : from importin beta regulation of nuclear pore assembly to identification of novel vertebrate proteins involved in mRNA export
The cell nucleus contains the genetic materials of an organism and allows selective nucleocytoplasmic transport via the nuclear pores embedded in the nuclear membranes. In interphase, importin beta is a well-studied nuclear import receptor mediating the classical nuclear import pathway. In mitosis, importin beta negatively regulates key mitotic events of mitotic spindle formation and nuclear pore assembly. Here, we showed that importin beta directly interacts with ELYS, the pore-targeting protein, and negatively interferes with the initiating step of nuclear pore assembly. We also showed that both Xenopus and human importin beta directly interacts with human centrin2, a small Ca-binding protein involved in centrosomes dynamics as well as protein and mRNA export. While nuclear import of transcription factors into the nucleus is required to initiate gene transcription, mRNA transcripts need to be exported out to the cytoplasm for translation into new proteins. In this study, we showed that both over-expression of human Shd1 and Eny2 causes nuclear accumulation of poly(A)+ RNA, suggesting potential roles in an unknown vertebrate TREX2 in mRNA export. Furthermore, Eny2 was shown to biochemically associate with multiple nuclear transport factors, notably TAP, the major mRNA export receptor. In conclusion, these data provide supporting evidence for a potential vertebrate TREX2 involved in mRNA export.