Biased Brownian motion as a mechanism to facilitate nanometer-scale exploration of the microtubule plus end by a kinesin-8

Abstract

Kinesin-8s are plus-end-directed motors that negatively regulate microtubule (MT) length. Well-characterized members of this subfamily (Kip3, Kif18A) exhibit two important properties: (i) They are "ultraprocessive," a feature enabled by a second MT-binding site that tethers the motors to a MT track, and (ii) they dissociate infrequently from the plus end. Together, these characteristics combined with their plus-end motility cause Kip3 and Kif18A to enrich preferentially at the plus ends of long MTs, promoting MT catastrophes or pausing. Kif18B, an understudied human kinesin-8, also limits MT growth during mitosis. In contrast to Kif18A and Kip3, localization of Kif18B to plus ends relies on binding to the plus-end tracking protein EB1, making the relationship between its potential plus-end-directed motility and plus-end accumulation unclear. Using single-molecule assays, we show that Kif18B is only modestly processive and that the motor switches frequently between directed and diffusive modes of motility. Diffusion is promoted by the tail domain, which also contains a second MT-binding site that decreases the off rate of the motor from the MT lattice. In cells, Kif18B concentrates at the extreme tip of a subset of MTs, superseding EB1. Our data demonstrate that kinesin-8 motors use diverse design principles to target MT plus ends, which likely target them to the plus ends of distinct MT subpopulations in the mitotic spindle.kinesin-8 | Kif18B | microtubule | EB1 | mitosis

Microtubule end tethering of a processive Kinesin-8 motor Kif18b is required for spindle positioning

“…Recent work proposed that Kif18b is a poorly processive motor which uses diffusion to reach microtubule ends, suggesting Kif18b tracks microtubule plus ends through a bind and release mechanism, similarly to other plus tip tracking proteins (Akhmanova and Steinmetz, 2008;Shin et al, 2015). We believe that a N-terminal GFP label may inhibit the motile properties of Kif18b, leading to its diffusive behavior (Shin et al, 2015).…”

“…Whether human Kinesin-8 Kif18a motor is a depolymerizing enzyme, a processive motor that dampens microtubule plus end dynamics, or both remains under debate (Mayr et al, 2007;Stumpff et al, 2008). A second human Kif18b, is reported to exhibit diffusion on the microtubule lattice using its C terminus and weak directed motility, which does not explain how it could target to or destabilise microtubule tips (Shin et al, 2015). Kif18b has previously been implicated in the negative regulation of astral microtubule length and has a modest contribution to chromosome alignment (Stout et al, 2011;Tanenbaum et al, 2011;Walczak et al, 2016).…”

“…Kif18b requires EB1 for microtubule end accumulation but the EBbinding motifs in Kif18b are not sufficient for plus tip localization (Tanenbaum et al, 2011). Additionally, Kif18b may precede EB1 at microtubule ends (Shin et al, 2015), suggesting other mechanisms enable Kif18b targeting to microtubule ends. Whether Kif18b cooperates with the Kinesin-13 microtubule depolymerase MCAK or independently depolymerizes microtubule ends also remains under debate (Tanenbaum et al, 2011;Walczak et al, 2016).…”

Processive Kinesin-14 HSET Exhibits Directional Flexibility Depending on Motor Traffic

“…The trapping laser in turned off, and the motion of the bead is then video-tracked with a DAGE CCD camera. Cross-correlation methods were used to track bead positions over time through custom MATLAB code [39,43,45]. The step/dwell finding algorithm was used to determine preference for motility in either direction.…”

Spatial regulation of astral microtubule dynamics by Kif18B in PtK cells

“…In contrast to Kif18A, much less is known about Kif18B in terms of both its activity and its function. Kif18B is also a plus end–directed motor, although it is far less processive than Kif18A ( Shin et al. , 2015 ).…”