Channel nuclear pore complex subunits are required for transposon silencing in Drosophila.

ABSTRACT

The nuclear pore complex (NPC) is the principal gateway between nucleus and cytoplasm that enables exchange of macromolecular cargo. Composed of multiple copies of ~30 different nucleoporins (Nups), the NPC acts as a selective portal, interacting with factors which individually license passage of specific cargo classes. Here we show that two Nups of the inner channel, Nup54 and Nup58, are essential for transposon silencing via the PIWI-interacting RNA (piRNA) pathway in the Drosophila ovary. In ovarian follicle cells, loss of Nup54 and Nup58 results in compromised piRNA biogenesis exclusively from the flamenco locus, whereas knockdowns of other NPC subunits have widespread consequences. This provides evidence that some Nups can acquire specialised roles in tissue-specific contexts. Our findings consolidate the idea that the NPC has functions beyond simply constituting a barrier to nuclear/cytoplasmic exchange as genomic loci subjected to strong selective pressure can exploit NPC subunits to facilitate their expression.

Transposons are genetic sequences, which, when active, can move around the genome and insert themselves into new locations. This can potentially disrupt the information required for cells to work properly in reproductive organs, for example, transposon activity can lead to infertility. Many organisms therefore have cellular systems that keep transposons in check. Animal cells comprise two main compartments the nucleus, which contains the genetic information, and the cytosol, where most chemical reactions necessary for life take place. Molecules continually move between nucleus and cytosol, much as people go in and out of a busy train station. The connecting 'doors' between the two compartments are called Nuclear Pore Complexes (NPCs), and their job is to ensure that each molecule passing through reaches its correct destination. Recent research shows that the individual proteins making up NPCs (called nucleoporins) may play other roles within the cell. In particular, genetic studies in fruit flies suggested that some nucleoporins help to control transposon activity within the ovary ­ but how they did this was still unclear. Munafò et al. therefore set out to determine if the nucleoporins were indeed actively silencing the transposons, or if this was just a side effect of altered nuclear-cytosolic transport. Experiments using cells grown from fruit fly ovaries revealed that depleting two specific nucleoporins, Nup54 and Nup58, re-activated transposons with minimal effects on most genes or the overall health of the cells. This suggests that Nup54 and Nup58 play a direct role in transposon silencing. Further, detailed analysis of gene expression in Nup54- and Nup58-lacking cells revealed that the product of one gene, flamenco, was indeed affected. Normally, flamenco acts as a 'master switch' to turn off transposons. Without Nup54 and Nup58, the molecule encoded by flamenco could not reach its dedicated location in the cytosol, and thus could not carry out its task. These results show that, far from being mere 'doorkeepers' for the nucleus, nucleoporins play important roles adapted to individual tissues in the body. Further research will help determine if the same is true for other organisms, and if these mechanisms can help understand human diseases.