Finding a right place to cut: How katanin is targeted to cellular severing sites

Abstract

Microtubule severing by katanin plays key roles in generating various array patterns of dynamic microtubules, while also responding to developmental and environmental stimuli. Quantitative imaging and molecular genetic analyses have uncovered that dysfunction of microtubule severing in plant cells leads to defects in anisotropic growth, division and other cell processes. Katanin is targeted to several subcellular severing sites. Intersections of two crossing cortical microtubules attract katanin, possibly by using local lattice deformation as a landmark. Cortical microtubule nucleation sites on preexisting microtubules are targeted for katanin-mediated severing. An evolutionary conserved microtubule anchoring complex not only stabilises the nucleated site, but also subsequently recruits katanin for timely release of a daughter microtubule. During cytokinesis, phragmoplast microtubules are severed at distal zones by katanin, which is tethered there by plant-specific microtubule-associated proteins. Recruitment and activation of katanin are essential for maintenance and reorganisation of plant microtubule arrays.

Keywords: katanin; microtubule severing; plant cells.

Graphical abstract

Fig. 1.

Katanin-mediated microtubule severing in interphase and cytokinesis. In interphase cells (left), katanin is localised at and severs nucleation sites and crossover sites of cortical microtubules, whereas in cytokinetic cells (right), katanin functions at a distal zone of expanding phragmoplasts. The plus (+) and minus (−) ends of microtubules are indicated.

Fig. 2.

Katanin-mediated microtubule severing at microtubule crossover sites. In plant interphase cells, crossovers are generated when two plasma membrane-associated cortical microtubules collide. Katanin accumulates at the crossover site by recognising elusive marks, possibly involving lattice defects, and severs an overriding microtubule. Since the precise geometry of microtubule crossover sites has not been clarified at the nanoscale level, the presented images are speculative and serve only for mechanistic discussions. The plus (+) and minus (−) ends of microtubules are indicated.

Fig. 3.

Katanin-mediated severing and release of a daughter microtubule after nucleation. In plant interphase cells, both the γ-tubulin complex and the augmin complex accumulate at the lattice of existing cortical microtubules, and nucleate a daughter microtubule. After nucleation, the Msd1–Wdr8 complex is required to stabilise the branching nucleation structure; its absence results in spontaneous detachment of a daughter microtubule (yellow panel). The Msd1–Wdr8 stabilising complex subsequently recruits katanin for regulated severing and release of a daughter microtubule. The plus (+) and minus (−) ends of microtubules are indicated.