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bioRxiv · 10.1101/2024.11.07.622534

Fishnet mesh of centrin-Sfi1 drives ultrafast calcium-activated contraction of the giant cell Spirostomum ambiguum

Lannan, J.·Floyd, C.·Xu, L. X.·Yan, C.·Marshall, W. F.·Vaikuntanathan, S.·Dinner, A. R.·Honts, J. E.·Bhamla, S.·Elting, M. W.
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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 remain 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 co-localize 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 support 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. SIGNIFICANCE STATEMENTMany 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 co-localize 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, ATP-independent bioin-spired actuators and synthetic cellular machinery.

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Lannan, J., Floyd, C., Xu, L. X., Yan, C., Marshall, W. F., Vaikuntanathan, S., Dinner, A. R., Honts, J. E., Bhamla, S., Elting, M. W.. 2024-11-08. Fishnet mesh of centrin-Sfi1 drives ultrafast calcium-activated contraction of the giant cell Spirostomum ambiguum. https://doi.org/10.1101/2024.11.07.622534

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