Selective nuclear export mechanism of small RNAs.

The timely nuclear-cytoplasmic translocation of proteins and RNAs by importins and exportins is important for controlling biological processes. Since the 2004 publication of the first exportin structure, Cse1p, the X-ray structures of exportin-5 complexed with pre-microRNA, exportin-t complexed with tRNA, and three CRM1-related structures have revealed the binding mechanism involved in specific cargo recognition. Pre-microRNA and tRNA have conserved 3' 2-4-nucleotide overhang motifs and similar short double-stranded regions. Exportin-5 and exportin-t bind a conserved 3' overhang strongly, and they weakly enclose the short double-stranded stems, each in a different manner. The structures of the nuclear export complexes of small double-stranded RNAs, pre-microRNAs, and tRNAs provide information about the specificities of the two exportins in the context of transcription and translation control.