A novel actin-microtubule cross-linking kinesin, NtKCH, functions in cell expansion and division

Kinesins with a calponin homology domain (KCHs) have been identified recently as a plant-specific subgroup of the kinesin-14 family and are suspected to act as microtubuleactin filament cross-linkers. The cellular function, however, has remained elusive. In order to address the function of KCHs, we isolated NtKCH, a novel KCH homologue from tobacco BY-2 cells. Following synchronization, NtKCH transcripts were shown to be abundant during mitosis, whereas, during interphase, expression was low. Using fluorescent-tagged cell lines and immunolabelling techniques, the localization of tobacco KCH was found to differ depending on the cell cycle. During interphase, NtKCH mainly associated with cortical microtubules, whereas a subfraction also co-localized with perinuclear actin cables. In dividing cells, NtKCH accumulated at the pre-prophase band and at the phragmoplast. However, it remained absent from spindle microtubules, but, instead, concentrated at two agglomerations in proximity to the two cell poles. This work develops a detailed model for the dual localization and function of NtKCH during cell division vs cell expansion. This model implies two dynamic states of KCHs that differ with regard to actin interaction. This allows the modulation of force generation by KCH in a cell cycle-dependent capture mechanism.