Biology subjects

Kikkawa, S.

Publications and source records attributed to Kikkawa, S..

3 recordsLinked to original sources

bioRxiv2025

Human spastin functions as an ATPase-independent microtubule nucleator via ring-stack formation

Microtubule severing enzyme spastin plays pivotal roles in cytokinesis and neuronal outgrowth, and its mutations cause hereditary spastic paraplegia (HSP). Here we show that, at physiological tubulin concentrations, human spastin behaves predominantly as an ATPase- independent microtubule nucleator rather than a severase. Biochemical and structural analyses revealed that spastin assembles tubulin into stacked-ring intermediates via strategically positioned microtubule-binding domains, thereby generating nucleation-competent sites for polymerization. Tubulin subunits within these spastin-induced rings adopt a straight, microtubule-like interface, in marked contrast to the twisted tubulin spirals characteristic of depolymerizing microtubule ends. Our results redefine the mechanistic landscape of multifaceted spastin functions and provide insights that may inform the pathological basis of HSP and guide future therapeutic strategies.

biophysics↗

bioRxiv2022

Structural model of microtubule dynamics inhibition by Kinesin-4 from the crystal structure of KLP-12 - tubulin complex

Kinesin superfamily proteins are microtubule-based molecular motors driven by the energy of ATP hydrolysis. Among them, the kinesin-4 family is a unique motor that inhibits microtubule dynamics. Although mutations of kinesin-4 cause several diseases, its molecular mechanism is unclear because of the difficulty of visualizing the high-resolution structure of kinesin-4 working at the microtubule plus-end. Here, we report that KLP-12, a C. elegans kinesin-4 ortholog of KIF21A and KIF21B, is essential for proper length control of C. elegans axons, and its motor domain represses microtubule polymerization in vitro. The crystal structure of the KLP-12 motor domain complexed with tubulin, which represents the high-resolution structural snapshot of inhibition state of microtubule-end dynamics, revealed the bending effect of KLP-12 for tubulin. Comparison with the KIF5B-tubulin and KIF2C-tubulin complexes, which represent the elongation and shrinking forms of microtubule ends, respectively, showed the curvature of tubulin introduced by KLP-12 is in between them. Taken together, KLP-12 controls the proper length of axons by modulating the curvature of the microtubule ends to inhibit the microtubule dynamics.

biophysics↗

bioRxiv2021

CAMSAP2 organizes a γ-tubulin-independent microtubule nucleation centre

Microtubules are dynamic polymers consisting of {beta}-tubulin heterodimers. The initial polymerization process, called microtubule nucleation, occurs spontaneously via {beta}-tubulin. Since a large energy barrier prevents microtubule nucleation in cells, the {gamma}-tubulin ring complex is recruited to the centrosome to overcome the nucleation barrier. However, detachment of a considerable number of microtubules from the centrosome is known to contribute to fundamental processes in cells. Here, we present evidence that minus-end-binding calmodulin-regulated spectrin-associated protein 2 (CAMSAP2) serves as a strong nucleator for microtubule formation from soluble {beta}-tubulin independent of {gamma}-tubulin. CAMSAP2 significantly reduces the nucleation barrier close to the critical concentration for microtubule polymerization by stabilizing the longitudinal contacts among {beta}-tubulins. CAMSAP2 clusters together with {beta}-tubulin to generate nucleation intermediates, from which numerous microtubules radiate, forming aster-like structures. Our findings suggest that CAMSAP2 supports microtubule growth by organizing a nucleation centre as well as by stabilizing microtubule nucleation intermediates.

cell biology↗