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CAMSAP2 Organizes A γ-tubulin-independent Microtubule Nucleation Centre

Tsuyoshi Imasaki, Satoshi Kikkawa, Shinsuke Niwa, Yumiko Saijo-Hamano, Hideki Shigematsu, Kazuhiro Aoyama, Kaoru Mitsuoka, Mari Aoki, Ayako Sakamoto, Yuri Tomabechi, Naoki Sakai, Mikako Shirouzu, Shinya Taguchi, Yosuke Yamagishi, Tomiyoshi Setsu, Yoshiaki Sakihama, Takahiro Shimizu, Eriko Nitta, Masatoshi Takeichi, Ryo Nitta

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AbstractMicrotubules are dynamic polymers consisting of αβ-tubulin heterodimers. The initial polymerization process, called microtubule nucleation, occurs spontaneously via αβ-tubulin. Since a large energy barrier prevents microtubule nucleation in cells, the γ-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 αβ-tubulin independent of γ-tubulin. CAMSAP2 significantly reduces the nucleation barrier close to the critical concentration for microtubule polymerization by stabilizing the longitudinal contacts among αβ-tubulins. CAMSAP2 clusters together with αβ-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.