Xenopus laevis Kif18A is a highly processive kinesin required for meiotic spindle integrity

Abstract

The assembly and functionality of the mitotic spindle depends on the coordinated activities of microtubule-associated motor proteins of the dynein and kinesin superfamily. Our current understanding of the function of motor proteins is significantly shaped by studies using Xenopus laevis egg extract as its open structure allows complex experimental manipulations hardly feasible in other model systems. Yet, the Kinesin-8 orthologue of human Kif18A has not been described in Xenopus laevis so far. Here, we report the cloning and characterization of Xenopus laevis (Xl) Kif18A. Xenopus Kif18A is expressed during oocyte maturation and its depletion from meiotic egg extract results in severe spindle defects. These defects can be rescued by wild-type Kif18A, but not Kif18A lacking motor activity or the C-terminus. Single-molecule microscopy assays revealed that Xl_Kif18A possesses high processivity, which depends on an additional C-terminal microtubule-binding site. Human tissue culture cells depleted of endogenous Kif18A display mitotic defects, which can be rescued by wild-type, but not tail-less Xl_Kif18A. Thus, Xl_Kif18A is the functional orthologue of human Kif18A whose activity is essential for the correct function of meiotic spindles in Xenopus oocytes.

Involvement of Kif4a in Spindle Formation and Chromosome Segregation in Mouse Oocytes

“…Furthermore, Kif14 can inhibit minus-end-directed microtubule motility [ 24 ], and Eg5 (also known as Kif11) affects spindle assembly and spindle bipolarity in Hela cells [ 25 ]. In Xenopus oocytes, Kif18a is indispensable for the correct function of meiotic spindles [ 26 ]. In Caenorhabditis elegans embryos, depletion of KLP-7 MCAK (a member of the kinesin-13 family) delays spindle and sister chromatid segregation and prevents assembly of astral microtubules [ 27 ].…”

Emerging Insights into the Function of Kinesin-8 Proteins in Microtubule Length Regulation

“…Kinesin-8 proteins are MT plus-end directed motors and plus-end MT destabilizing enzymes (Table 2). Most Kinesin-8 proteins are fairly slow motors (~3 µm/min), and many are highly processive, traveling ~10 µm before dissociating from the MT [31,68,70]. Processivity allows motor proteins to travel long distances in cells, and for Kinesin-8 proteins it may be important to help them reach the ends of MTs in the spindle.…”

“…Processivity allows motor proteins to travel long distances in cells, and for Kinesin-8 proteins it may be important to help them reach the ends of MTs in the spindle. Kif18A is approximately 5-fold faster than the other Kinesin-8 proteins, while still maintaining high processivity [68,70]. Not all Kinesin-8 proteins are highly processive, as Kif18B was initially reported to be much less processive due to switching of the motor between a diffusive state and directed motility until it reached the MT end, where it dwells at the MT end [71].…”

Structural basis of human kinesin-8 function and inhibition

“…Klp5/6 can both couple cargo movement to MT depolymerization ( 22 ), and have also been shown to enhance MT nucleation and induce catastrophe at MT ends ( 23 ). Although vertebrate kinesin-8s all display plus-end directed motility with robust processivity ( 16 , 24 – 26 ), reports concerning their ability to modulate MT dynamics vary. For dimeric Kif18A, both active depolymerization ( 11 ) and antagonism of MT assembly have been reported ( 25 , 27 , 28 ).…”