RNP EXPORT IS MEDIATED BY STRUCTURAL REORGANIZATION OF THE NUCLEAR-PORE BASKET

Messenger RNA leaves the cell nucleus as ribonucleoprotein (RNP) parti cles. The nucleocytoplasmic translocation of the particles takes place through the nuclear pure complex (NPC) and includes two steps: bindin g to the NPC and transit through its central channel. The NPC basket i s a fishtrap-like component of NPC facing the nucleoplasm. Its positio n in the NPC strongly suggests that it has an important role in the in itial steps of macromolecular export from the nucleus. Here we report a cyclic rearrangement. of tire basket structure in relation to the tr anslocation of a specific messenger RNP (mRNP) of exceptional size, th e Balbiani ring RNP particles in the salivary gland cells in Chironomu s. We used field emission in-lens scanning electron microscopy (FEISEM ), transmission electron microscopy (TEM), and immunocytochemistry to analyse the structural organization ol the basket during the mRNP expo rt. Our observations reveal five configurations of the basket which ar e presented in a model of basket reorganization related to the state o f mRNP penetration into the NPC. We suggest that the functional role o f the basket is to anchor the mRNP particle to the NPC and position it in correct oriental-ion at the entrance to the central channel of the NPC. (C) 1996 Academic Press Limited