MNK1 kinase activity is required for abscission.

MNK1 is a serine/threonine kinase identified as a target for MAP kinase pathways. Using chemical drug, kinase-dead expression or knockdown by RNA interference, we show that inhibition of MNK1 induces the formation of multinucleated cells, which can be rescued by expressing a form of MNK1 that is resistant to RNA interference. We found that the active human form of MNK1 localises to centrosomes, spindle microtubules and the midbody. Time-lapse recording of MNK1-depleted cells displays cytokinesis defects, as daughter cells fuse back together. When MNK1 activity was inhibited, no microtubule defect at the midbody was detected, however, anchorage of the membrane vesicle at the midbody was impaired as lumenal GFP-positive vesicles did not accumulate at the midbody. At the molecular level, we found that centriolin localisation was impaired at the midbody in MNK1-depleted cells. As a consequence, endobrevin - a v-SNARE protein implicated in the abscission step - was not properly localised to the midbody. Altogether, our data show that MNK1 activity is required for abscission.

Localization of aurora A and aurora B kinases during interphase: role of the N-terminal domain.

Aurora kinases possess a conserved catalytic domain (CD) and a N-terminal domain (ND) that varies in size and sequence. We have previously reported that the N-terminal domain of AuroraA (AurA) participates in the localization of the kinase to the centrosome in interphase. AuroraB (AurB) is a chromosome passenger protein and its N-terminal domain is not necessary for its localization or function during mitosis. Using various combinations of GFP-AurA and AurB protein domains we show that AurB N-terminal domain is required for nuclear localization in Xenopus XL2 cells in interphase. In human cells, however, we found both AurA and AurB kinases in the nucleus, AurA being mainly cytoplasmic and AurB mainly nuclear. Both proteins are actively excluded from the nucleus by a CRM1 dependent pathway. Interestingly, at a functional level, in interphase, every combination of Aurora kinase domains (ND-CD) rescues histone H3 Serine10 phosphorylation defect induced by AurB knockdown. This clearly indicates the presence of a functional AurA in the nucleus. However, the chimera ND-AurA/CD-AurB was much more efficient than the ND-AurB/ CD-AurA to rescue multinucleation also induced by AurB knockdown. This indicates that the catalytic domain of AurB is required to fulfill specific functions during mitosis that cannot be fulfilled by the catalytic domain of AurA, probably for localization reasons during mitosis.