INCYTES from MBC

Shuttling nuclear transport receptors mediate nucleocytoplasmic traffic through nuclear pore complexes (NPCs). Importin , the best studied receptor, is also a key regulator of cell cycle events from mitotic spindle assembly to nuclear envelope fusion and NPC assembly. Now, Lau, Delmar et al. show that a second import receptor, transportin, regulates the same set of mitotic assembly events, including spindle assembly. Both transportin and importin are seen to negatively regulate the earliest known step in NPC assembly, the seeding of chromatin with the critical proteins ELYS and the Nup107-160 complex. Indeed, the two import receptors bind directly to the C-terminus of ELYS. Rotem, Gruber, Shorer et al. focus on the chromatin seeding step and its regulation by importin . Importin is shown to form a high molecular weight complex with both ELYS and Nup107-160, preventing them from binding chromatin. Seeding sites consisting of ELYS and the Nup107-160 complex are formed along the topmost ridges of the chromatin landscape and can be visualized by immunolabeling and high-resolution scanning electron microscopy.