RNP export is mediated by structural reorganization of the nuclear pore basket

Messenger RNA leaves the cell nucleus as ribonucleoprotein (RNP) particles. The nucleocytoplasmic translocation of the particles takes place through the nuclear pore complex (NPC) and includes two steps: binding to the NPC and transit through its central channel. The NPC basket is a fishtrap-like component of NPC facing the nucleoplasm. Its position in the NPC strongly suggests that it has an important role in the initial steps of macromolecular export from the nucleus. Here we report a cyclic rearrangement of the basket structure in relation to the translocation of a specific messenger RNP (mRNP) of exceptional size, the Balbiani ring RNP particles in the salivary gland cells in Chironomus. We used field emission in-lens scanning electron microscopy (FEISEM), transmission electron microscopy (TEM), and immunocytochemistry to analyse the structural organization of the basket during the mRNP export. Our observations reveal five configurations of the basket which are presented in a model of basket reorganization related to the state of mRNP penetration into the NPC. We suggest that the functional role of the basket is to anchor the mRNP particle to the NPC and position it in correct orientation at the entrance to the central channel of the NPC.