Microtubule regulation in the moss Physcomitrella patens

Leong et al. examine the roles of Kinesin-13 and -8 in moss mitosis.

Plant Cell https://doi.org/10.1105/tpc.19.00521

By Shu Yao Leong (Nagoya University, currently Eberhard Karls University of Tübingen) & Moé Yamada (Nagoya University)

Background: Microtubules (MTs) are dynamic macromolecular protein filaments of the cytoskeletal network that can undergo assembly and disassembly repeatedly. Regulation of MT dynamics is essential to its function in many different cellular processes, such as cell growth and cell division. Kinesin-13 and Kinesin-8 are well-known MT depolymerases in animal cells, reported to induce severe mitotic defects upon protein depletion. While little is known about plant Kinesin-8, rice and Arabidopsis Kinesin-13 have been shown to have MT depolymerising activity. However, complete depletion of plant Kinesin-13 and -8 have yet to be studied, with their function during mitosis largely unknown.

Question: What are Kinesin-13 and Kinesin-8 doing in plants, or more specifically, in mitosis?

Findings: Complete knockout (KO) plants of Kinesin-13 and -8 in the moss Physcomitrella patens were generated, indicating that both kinesins are not essential for viability. While the Kinesin-8 KO line showed no observable mitotic defects, the Kinesin-13 KO line showed mild mitotic defects, such as reduced spindle length, increased mitotic duration, and retrograde nuclear movement during prophase. However, neither of the kinesin KO lines showed the dramatic mitotic phenotypes observed in animal cells. Moreover, MT depolymerisation activity could not be verified for either kinesin. In in vitro assays, the Kinesin-13 motor domain instead showed mild MT catastrophe-inducing activity, and the Kinesin-8 motor domain showed MT gliding activity. Interestingly, both kinesin KO lines had wavy protonema filaments, which correlated with unstable positioning of the MT foci at the tip of these filaments. These results suggest that plant Kinesin-13 and -8 have acquired different intrinsic molecular activity and functions.

Next steps: Depletion of either Kinesin-13 or Kinesin-8 does not result in aberrant mitosis, suggesting that a unique mechanism underlies plant mitotic spindle formation. It would be interesting to study how plant cells regulate spindle dynamics without both kinesins.

Shu Yao Leong, Tomoya Edzuka, Gohta Goshima, and *Moé Yamada. (2020). Kinesin-13 and Kinesin-8 Function During Cell Growth and Division in the Moss Physcomitrella patens. Plant Cell; DOI: https://doi.org/10.1105/tpc.19.00521

Key words: microtubule, Kinesin-13, Kinesin-8, Physcomitrella patens