The giant single-celled eukaryote, Spirostomum , exhibits one of the fastest movements in the biological world. This ultrafast contraction is dependent on Ca 2+ rather than ATP and therefore differs to the actin-myosin system in muscle. We obtained the high-quality genome of Spirostomum minus from which we identified the key molecular components of its contractile apparatus, including two major Ca 2+ binding proteins (Spasmin 1 and 2) and two giant proteins (GSBP1 and GSBP2), which act as the backbone and allow for the binding of hundreds of spasmins. The evidence suggests that the GSBP-spasmin protein complex is the functional unit of the mesh-like contractile fibrillar system, which, coupled with various other subcellular structures, provides the mechanism for repetitive ultrafast cell contraction and extension. These findings improve our understanding of the Ca 2+ -dependent ultrafast movement and provide a blueprint for future biomimicry, design, and construction of this kind of micromachine.

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