2026-06-26 ノースカロライナ州立大学（NC State）
Spirostomum ambiguum. Image: Joseph Lannan
＜関連情報＞
- https://news.ncsu.edu/2026/06/this-tiny-organism-contracts-200-times-faster-than-we-can-blink-heres-how/
- https://www.pnas.org/doi/10.1073/pnas.2601408123
セントリン-Sfi1ミオネームの網状構造が、巨大繊毛虫Spirostomum ambiguumにおける超高速カルシウム誘発性収縮を促進する A centrin–Sfi1 myoneme fishnet powers ultrafast calcium-triggered contraction in the giant ciliate Spirostomum ambiguum
Joseph Lannan, Carlos Floyd, L. X. Xu, +7 , and Mary Williard Elting
Proceedings of the National Academy of Sciences Published:May 29, 2026
DOI:https://doi.org/10.1073/pnas.2601408123
Significance
Many cells change shape using actomyosin, but some protists contract using calcium-activated protein networks called myonemes. We combine quantitative imaging, electron microscopy, multiscale modeling, and in vitro reconstitution to link molecular-scale mechanisms to the millisecond shortening of the giant ciliate Spirostomum. Centrin and an Sfi1 homolog colocalize in a fishnet-like cortical mesh, and simulations show that this geometry can reproduce the observed whole-cell shape change under volume conservation. Purified centrin–Sfi1 complexes undergo calcium-dependent compaction and self-association, supporting a protein-scale switch that can drive myoneme contraction. These results connect calcium signaling to whole-cell mechanics and suggest principles for designing fast, adenosine triphosphate (ATP)-independent bioinspired actuators and synthetic cellular machinery.
Abstract
Spirostomum is a giant unicellular ciliate that contracts to a quarter of its body length in less than five milliseconds, achieving an order of magnitude higher fractional shortening rate than actomyosin-based systems. This ultrafast contraction is powered by myonemes, calcium-activated protein networks at the cortex whose biochemical mechanism remains unclear. We quantify changes in cortical microtubules, membrane ruffles, and the fishnet-like myoneme mesh during contraction, and develop multiscale models that connect local myoneme shortening to whole-cell shape change. Centrin and an Sfi1 homolog colocalize with the myoneme by immunofluorescence and localize to the myoneme by immunogold electron microscopy. Coarse-grained mesh simulations reproduce the measured deformations and show that fishnet geometry, together with volume conservation, leads to uniform contraction. Finally, we reconstitute a Spirostomum centrin–Sfi1 repeat complex in vitro and measure calcium-dependent compaction and self-association, supporting a molecular basis for myoneme contractility. Together, these results underpin a multiscale model in which calcium-responsive centrin–Sfi1 structures are the central contractile element in Spirostomum and suggest design principles for fast, calcium-triggered, chemomechanical contractile networks that operate without actomyosin or ATP.
