Discrete regions of the kinesin-8 Kip3 tail differentially mediate astral microtubule stability and spindle disassembly

Single depolymerizing and transport kinesins stabilize microtubule ends

Microtubules are highly dynamic cellular filaments and an accurate control of their length is important for many intracellular processes like cell division. Among other factors, microtubule length is actively modulated by motors from the kinesin superfamily. For example, yeast kinesin-8, Kip3, motors depolymerize microtubules by a cooperative, force- and length-dependent mechanism. However, whether single motors can also depolymerize microtubules is unclear. Here, we measured how single kinesin motors influenced the stability of microtubules in an in vitro assay. Using label-free interference reflection microscopy, we determined the spontaneous microtubule depolymerization rate of stabilized microtubules in the presence of kinesins. Surprisingly, we found that both single Kip3 and nondepolymerizing kinesin-1 transport motors, used as a control, stabilized microtubules further. For Kip3, this behavior is contrary to the collective force-dependent depolymerization activity of multiple motors. Because of the control measurement, the finding may hint at a more general stabilization mechanism. The complex, concentration-dependent interaction with microtubule ends provides new insights into the molecular mechanism of kinesin-8 and its regulatory function of microtubule length.

Kinesin-8 motors: regulation of microtubule dynamics and chromosome movements

Microtubules are essential for intracellular transport, cell motility, spindle assembly, and chromosome segregation during cell division. Microtubule dynamics regulate the proper spindle organization and thus contribute to chromosome congression and segregation. Accumulating studies suggest that kinesin-8 motors are emerging regulators of microtubule dynamics and organizations. In this review, we provide an overview of the studies focused on kinesin-8 motors in cell division. We discuss the structures and molecular kinetics of kinesin-8 motors. We highlight the essential roles and mechanisms of kinesin-8 in the regulation of microtubule dynamics and spindle organization. We also shed light on the functions of kinesin-8 motors in chromosome movement and the spindle assembly checkpoint during the cell cycle.

The quantification and regulation of microtubule dynamics in the mitotic spindle

Microtubules play essential roles in cellular organization, cargo transport, and chromosome segregation during cell division. During mitosis microtubules form a macromolecular structure known as the mitotic spindle that is responsible for the accurate segregation of chromosomes between the two daughter cells. This is accomplished thanks to finely tuned control of microtubule dynamics. Even small changes in microtubule dynamics during spindle formation and/or operation may lead to chromosome mis-segregation, chromosome instability and aneuploidy. These three events are directly correlated with human diseases like cancer and developmental defects. Precise measurements of microtubule dynamics in the spindle will allow us to discover new molecules involved in regulating microtubule dynamics and enable a deeper understanding of the mechanisms that underlie mitosis and cancer emergence and development. Moreover, many chemotherapeutic agents for cancer treatment are targeted to microtubules, so continued investigation of their dynamics with utmost precision will facilitate the development of new drugs. Measuring microtubule dynamics in the spindle has been a difficult task until recently. With the development of new and gentler microscopic techniques, and new computer programs, we can perform better and more accurate measurements of microtubule dynamics during mitosis.

A unified model for microtubule rescue

How microtubules transition from depolymerization to polymerization, known as rescue, is poorly understood. Here we examine two models for rescue: 1) an "end-driven" model in which the depolymerizing end stochastically switches to a stable state; and 2) a "lattice-driven" model in which rescue sites are integrated into the microtubule before depolymerization. We test these models using a combination of computational simulations and in vitro experiments with purified tubulin. Our findings support the "lattice-driven" model by identifying repeated rescue sites in microtubules. In addition, we discover an important role for divalent cations in determining the frequency and location of rescue sites. We use "wash-in" experiments to show that divalent cations inhibit rescue during depolymerization, but not during polymerization. We propose a unified model in which rescues are driven by embedded rescue sites in microtubules, but the activity of these sites is influenced by changes in the depolymerizing ends.

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

Proper regulation of microtubules (MTs) is critical for the execution of diverse cellular processes, including mitotic spindle assembly and chromosome segregation. There are a multitude of cellular factors that regulate the dynamicity of MTs and play critical roles in mitosis. Members of the Kinesin-8 family of motor proteins act as MT-destabilizing factors to control MT length in a spatially and temporally regulated manner. In this review, we focus on recent advances in our understanding of the structure and function of the Kinesin-8 motor domain, and the emerging contributions of the C-terminal tail of Kinesin-8 proteins to regulate motor activity and localization.