The Nup155-mediated organisation of inner nuclear membrane proteins is independent of Nup155 anchoring to the metazoan nuclear pore complex

The nuclear envelope (NE), an important barrier between the nucleus and the cytoplasm, is composed of three structures: the outer nuclear membrane, which is continuous with the ER, the inner nuclear membrane (INM), which interfaces with chromatin, and nuclear pore complexes (NPCs), which are essential for the exchange of macromolecules between the two compartments. The NPC protein Nup155 has an evolutionarily conserved role in the metazoan NE formation; but the in vivo analysis of Nup155 has been severely hampered by the essential function of this protein in cell viability. Here, we take advantage of the hypomorphicity of RNAi systems and use a combination of protein binding and rescue assays to map the interaction sites of two neighbouring NPC proteins Nup93 and Nup53 on Nup155, and to define the requirements of these interactions in INM protein organization. We show that different parts of Drosophila Nup155 have distinct functions: the Nup155 beta-propeller anchors the protein to the NPC, whereas the alpha-solenoid part of Nup155 is essential for the correct localisation of INM proteins lamin-B receptor (LBR) and otefin. Using chromatin extracts from semisynchronized cells, we also provide evidence that the Nup155 alpha-solenoid has a chromatin-binding activity that is stronger at the end of mitosis. Our results argue that the role of Nup155 in INM protein localisation is not mediated through the NPC anchoring activity of the protein and suggest that regions other than Nup155 beta-propeller are necessary for the targeting of proteins to the INM.