In today’s global economy, imports and exports can ebb and flow dramatically depending on economic conditions. But in the fine-tuned cellular economy, where supply-demand runs the show, miRNA Importin and Exportin operate with consistency that would have most economists envious.
When pre-miRNAs are shipped from the nucleus to the cytoplasm. Exportin-5 escorts the ~65 nt dsRNAs through the nuclear pore, protecting it from harmful nucleases that would cause miRNA damage while en-route to the delivery destination.
A team of Japanese researchers got a peek inside the pre-miR ‘exportin’ business when they solved the crystal structure of pre-miRNA complexed with Exp-5 and a guanine triphosphatase (Ran GTP) at 2.9 Å resolution. ??
They found the double-stranded stem snuggled cozily inside a “baseball mitt-like” pocket, protected on four sides from endonucleases. Similarly, the 3′ overhang sticks into a “tunnel-like” opening, shielding it (and the 5′ end) from exonucleases. And since it’s the phosphate backbone — and not the bases themselves — that interacts with Exp-5:Ran GTP, pre-miRNA sequence doesn’t seem to matter.??
As it turns out, the Exp-5 complex is also something of a ‘one-size-fits-all’ shipping container. The authors noticed that it can handle several other species of RNA as well.
See how the importin-β family member, Exp-5 complex keeps its contents safe at Science, November 2009.