编者按：这是个有趣的科学问题。今天出版的Science发表的文章揭示这个机理。miRNA和siRNA已广泛应用，但如何从核中进入胞浆以发挥作用？这是值得思考的问题，这篇paper显示Exp5蛋白可能发挥关键性作用。这为miRNA的机理揭示和应用的认识更进一步！
Nuclear Export of MicroRNA Precursors
Elsebet Lund,1 Stephan Güttinger,2 Angelo Calado,2* James E. Dahlberg,1 Ulrike Kutay2
Science, Vol. 303, Issue 5654, 95-98, January 2, 2004
MicroRNAs (miRNAs), which function as regulators of gene expression in eukaryotes, are processed from larger transcripts by sequential action of nuclear and cytoplasmic ribonuclease III–like endonucleases. We show that Exportin-5 (Exp5) mediates efficient nuclear export of short miRNA precursors (pre-miRNAs) and that its depletion by RNA interference results in reduced miRNA levels. Exp5 binds correctly processed pre-miRNAs directly and specifically, in a Ran guanosine triphosphate–dependent manner, but interacts only weakly with extended pre-miRNAs that yield incorrect miRNAs when processed by Dicer in vitro. Thus, Exp5 is key to miRNA biogenesis and may help coordinate nuclear and cytoplasmic processing steps.
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