Abstract
Spirostomum is a unicellular ciliate capable of contracting to a quarter of its body length in less than five milliseconds. When measured as fractional shortening, this is an order of magnitude faster than motion powered by actomyosin. Myonemes, which are protein networks found near the cortex of many protists, are believed to power Spirostomum contraction. Fast contraction, slow elongation, and calcium-triggering are hallmarks of myoneme-based motion. The biochemical basis of this motion and the molecular mechanism that supports such fast speeds are not well understood. Previous work suggests that myoneme structures in some protists are rich in centrin and Sfi1 homologs, two proteins that may underlie contraction. Centrin undergoes a significant conformational change in the presence of calcium, allowing it to bind to other centrin molecules. To understand Spirostomum contraction, we measure changes in cortical structures and model contraction of the whole cell and of the underlying protein complexes. We provide evidence that centrin/Sfi1 structures are responsible for contraction, which we propose is powered by a modulated entropic spring. Using this model, we recapitulate organismal-scale contraction in mesh simulation experiments and demonstrate the importance of structural organization of myoneme in a fishnet-like structure. These results provide a cohesive, multiscale model for the contraction of Spirostomum. Deeper understanding of how single cells can execute extreme shape changes holds potential for advancing cell biophysics, synthetically engineering contractile machinery, and cellular-inspired engineering designs.