Exportin Crm 1 is repurposed as a docking protein to generate 1 microtubule organizing centers at the nuclear pore 2 3 4

27 28 Non-centrosomal microtubule organizing centers (MTOCs) are important for microtubule 29 organization in many cell types. In fission yeast Schizosaccharomyces pombe, the protein 30 Mto1, together with partner protein Mto2 (Mto1/2 complex), recruits the -tubulin complex to 31 multiple non-centrosomal MTOCs, including the nuclear envelope (NE). Here, we develop a 32 comparative-interactome mass spectrometry approach to determine how Mto1 localizes to 33 the NE. Surprisingly, we find that Mto1, a constitutively cytoplasmic protein, docks at nuclear 34 pore complexes (NPCs), via interaction with exportin Crm1 and cytoplasmic FG-nucleoporin 35 Nup146. Although Mto1 is not a nuclear export cargo, it binds Crm1 via a nuclear export 36 signal-like sequence, and docking requires both Ran in the GTP-bound state and Nup146 37 FG repeats. In addition to determining the mechanism of MTOC formation at the NE, our 38 results reveal a novel role for Crm1 and the nuclear export machinery in the stable docking 39 of a cytoplasmic protein complex at NPCs. 40