Distinct Interaction Modes of the Kinesin-13 Motor Domain with the Microtubule

Choreographing the motor-driven endosomal dance

The endosomal system orchestrates the transport of lipids, proteins and nutrients across the entire cell. Along their journey, endosomes mature, change shape via fusion and fission, and communicate with other organelles. This intriguing endosomal choreography, which includes bidirectional and stop-and-go motions, is coordinated by the microtubule-based motor proteins dynein and kinesin. These motors bridge various endosomal subtypes to the microtubule tracks thanks to their cargo-binding domain interacting with endosome-associated proteins, and their motor domain interacting with microtubules and associated proteins. Together, these interactions determine the mobility of different endosomal structures. In this Review, we provide a comprehensive overview of the factors regulating the different interactions to tune the fascinating dance of endosomes along microtubules.

Cryo-EM reveals the structural basis of microtubule depolymerization by kinesin-13s

Kinesin-13s constitute a distinct group within the kinesin superfamily of motor proteins that promote microtubule depolymerization and lack motile activity. The molecular mechanism by which kinesin-13s depolymerize microtubules and are adapted to perform a seemingly very different activity from other kinesins is still unclear. To address this issue, here we report the near atomic resolution cryo-electron microscopy (cryo-EM) structures of Drosophila melanogaster kinesin-13 KLP10A protein constructs bound to curved or straight tubulin in different nucleotide states. These structures show how nucleotide induced conformational changes near the catalytic site are coupled with movement of the kinesin-13-specific loop-2 to induce tubulin curvature leading to microtubule depolymerization. The data highlight a modular structure that allows similar kinesin core motor-domains to be used for different functions, such as motility or microtubule depolymerization.

Kinesin-13s are microtubule depolymerases that lack motile activity. Here the authors present the cryo-EM structures of kinesin-13 microtubule complexes in different nucleotide bound states, which reveal how ATP hydrolysis is linked to conformational changes and propose a model for kinesin induced depolymerisation.

Use of Single Molecule Fluorescence Polarization Microscopy to Study Protein Conformation and Dynamics of Kinesin-Microtubule Complexes

Single molecule fluorescence polarization microscopy (smFPM) is a technique that enables to monitor changes in the orientation of a single labeled protein domain. Here we describe a smFPM microscope set-up and protocols to investigate conformational changes associated with the movement of motor proteins along cytoskeletal tracks.

Microtubule Tip Tracking by the Spindle and Kinetochore Protein Ska1 Requires Diverse Tubulin-Interacting Surfaces

The macromolecular kinetochore functions to generate interactions between chromosomal DNA and spindle microtubules [1]. To facilitate chromosome movement and segregation, kinetochores must maintain associations with both growing and shrinking microtubule ends. It is critical to define the proteins and their properties that allow kinetochores to associate with dynamic microtubules. The kinetochore-localized human Ska1 complex binds to microtubules and tracks with depolymerizing microtubule ends [2]. We now demonstrate that the Ska1 complex also autonomously tracks with growing microtubule ends in vitro, a key property that would allow this complex to act at kinetochores to mediate persistent associations with dynamic microtubules. To define the basis for Ska1 complex interactions with dynamic microtubules, we investigated the tubulin-binding properties of the Ska1 microtubule binding domain. In addition to binding to the microtubule lattice and dolastatin-induced protofilament-like structures, we demonstrate that the Ska1 microtubule binding domain can associate with soluble tubulin heterodimers and promote assembly of oligomeric ring-like tubulin structures. We generated mutations on distinct surfaces of the Ska1 microtubule binding domain that disrupt binding to soluble tubulin but do not prevent microtubule binding. These mutants display compromised microtubule tracking activity in vitro and result in defective chromosome alignment and mitotic progression in cells using a CRISPR/Cas9-based replacement assay. Our work supports a model in which multiple surfaces of Ska1 interact with diverse tubulin substrates to associate with dynamic microtubule polymers and facilitate optimal chromosome segregation.

Dynamics of microtubules: highlights of recent computational and experimental investigations

Microtubules are found in most eukaryotic cells, with homologs in eubacteria and archea, and they have functional roles in mitosis, cell motility, intracellular transport, and the maintenance of cell shape. Numerous efforts have been expended over the last two decades to characterize the interactions between microtubules and the wide variety of microtubule associated proteins that control their dynamic behavior in cells resulting in microtubules being assembled and disassembled where and when they are required by the cell. We present the main findings regarding microtubule polymerization and depolymerization and review recent work about the molecular motors that modulate microtubule dynamics by inducing either microtubule depolymerization or severing. We also discuss the main experimental and computational approaches used to quantify the thermodynamics and mechanics of microtubule filaments.

Insight into microtubule disassembly by kinesin-13s from the structure of Kif2C bound to tubulin

Kinesin-13s are critical microtubule regulators which induce microtubule disassembly in an ATP dependent manner. To clarify their mechanism, we report here the crystal structure of a functional construct of the kinesin-13 Kif2C/MCAK in an ATP-like state and bound to the αβ-tubulin heterodimer, a complex mimicking the species that dissociates from microtubule ends during catalytic disassembly. Our results picture how Kif2C stabilizes a curved tubulin conformation. The Kif2C α4-L12-α5 region undergoes a remarkable 25° rotation upon tubulin binding to target the αβ-tubulin hinge. This movement leads the β5a–β5b motif to interact with the distal end of β-tubulin, whereas the neck and the KVD motif, two specific elements of kinesin-13s, target the α-tubulin distal end. Taken together with the study of Kif2C mutants, our data suggest that stabilization of a curved tubulin is an important contribution to the Kif2C mechanism.

Kinesin-13s are microtubule depolymerizing enzymes. Here the authors present the crystal structure of a DARPin fused construct comprising the short neck region and motor domain of kinesin-13 in complex with an αβ-tubulin heterodimer, which shows that kinesin-13 functions by stabilizing a curved tubulin conformation.