Persistent contacts between Climp63 and microtubules cause mitotic defects and nuclear fragmentation

The endoplasmic reticulum (ER) is the most elaborate endomembrane system in mammalian cells. To generate its characteristic shape, ER-resident proteins directly affect membrane topology, while interactions with the microtubule (MT) network determine ER positioning. The ER undergoes continuous remodeling with the most drastic rearrangements taking place during mitosis. A critical aspect is the reconfiguration of MT-ER interactions that accompanies spindle formation. One abundant ER-MT tether is constituted by the ER sheet-associated membrane protein Climp63, which binds MTs in a phosphorylation-sensitive manner. Here, we demonstrate that expression of a phosphodeficient Climp63 mutant in HeLa cells prevents dissolving of ER-MT contacts in mitosis, resulting in severe mitotic delays. Moreover, during mitotic exit, cells fail to properly enclose all chromosomes into a single nucleus, leading to excessive nuclear fragmentation. The emerging micronuclei assemble NE components with nuclear pore complexes, in contrast to what is known for micronuclei formed from lagging chromosomes. We further show that the N-terminal 28 amino acids are sufficient for the interaction between Climp63 and MTs with phosphorylation of serine S17 by CDK1 being critical for the mitotic release of the ER from MTs. Overall, our results demonstrate that aberrant Climp63 activity severely affects mitosis. One may speculate that mitotic aberrations may contribute to the poor prognosis associated with Climp63 overexpressing cancers.