The Kinesin ATK5 Functions in Early Spindle Assembly inArabidopsis 

Abstract

During cell division, the mitotic spindle partitions chromosomes into daughter nuclei. In higher plants, the molecular mechanisms governing spindle assembly and function remain largely unexplored. Here, live cell imaging of mitosis in Arabidopsis thaliana plants lacking a kinesin-14 (ATK5) reveals defects during early spindle formation. Beginning during prophase and lasting until late prometaphase, spindles of atk5-1 plants become abnormally elongated, are frequently bent, and have splayed poles by prometaphase. The period of spindle elongation during prophase and prometaphase is prolonged in atk5-1 cells. Time-lapse imaging of yellow fluorescent protein:ATK5 reveals colocalization with perinuclear microtubules before nuclear envelope breakdown, after which it congresses inward from the poles to the midzone, where it becomes progressively enriched at regions of overlap between antiparallel microtubules. In vitro microtubule motility assays demonstrate that in the presence of ATK5, two microtubules encountering one another at an angle can interact and coalign, forming a linear bundle. These data indicate that ATK5 participates in the search and capture of antiparallel interpolar microtubules, where it aids in generating force to coalign microtubules, thereby affecting spindle length, width, and integrity.

Endogenous localizome identifies 43 mitotic kinesins in a plant cell

“…The kinesins previously studied in other plant species, such as Arabidopsis or tobacco, generally show similar localization [e.g., kinesin 7-II (43, 44); kinesin 12-II (45); kinesin 14-I ( [46][47][48]]. This similarity may not be surprising, given that essential features of the mitotic apparatus, such as a lack of centrosomes, are conserved between mosses and flowering plants (14,23,49,50).…”

Two Microtubule-associated Proteins of Arabidopsis MAP65s Promote Antiparallel Microtubule Bundling

“…Janson et al (2007) demonstrated that Ase1p localizes to overlapping (Ϫ) ends during interphase and overlapping plus ends in the anaphase spindle, the difference in localization being determined by molecular motors as Klp2p (a minus-end-directed motor) mediates the sliding of newly nucleated MTs along antiparallel MT bundles. In plant cells, ATK5, a plus end tracking protein molecular motor (Ambrose et al, 2005) preferentially localizes at regions of overlap between opposing interpolar MTs, suggesting an increased affinity for plus ends in an antiparallel orientation (Ambrose and Cyr, 2007). In vitro, ATK5 equally coaligns with parallel or antiparallel MTs.…”

Emerging molecular mechanisms that power and regulate the anastral mitotic spindle of flowering plants

“…In the loss-of-function mutants, atk1 and atk5, mitosis is prolonged and the spindles are somewhat broader than in the wild type [Marcus et al, 2003;Ambrose et al, 2005;Ambrose and Cyr, 2007]. In atk5, prophase spindles are longer than those of the wild type [Ambrose and Cyr, 2007], and in atk1, prophase spindles have weak or even absent bipolarity, implying that these kinesins act early in mitosis and that forming the prophase spindle involves a force balance.…”

“…In atk5, prophase spindles are longer than those of the wild type [Ambrose and Cyr, 2007], and in atk1, prophase spindles have weak or even absent bipolarity, implying that these kinesins act early in mitosis and that forming the prophase spindle involves a force balance. In atk1, the metaphase spindle phenotype is more pronounced in meiotic cells [Chen et al, 2002;Marcus et al, 2003], resembling the polar disruption seen in Drosophila ncd mutants [Matthies et al, 1996], and is sufficiently severe as to reduce male fertility [Chen et al, 2002].…”