Ultrafast contraction: Investigating the structures underlying myoneme force generating networks in Spirostomum sp.

Spirostomum sp. 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 networks of proteins found near the cortex of many protists, are thought to power this 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 allows motion to occur at such fast speeds, are not well understood. Previous work suggests myoneme structures in some protists are rich in centrin, a protein that may underlie contraction. Centrin undergoes a significant conformational change in the presence of calcium allowing it to bind to other centrin molecules. We use transmission electron microscopy to image Spirostomum in both contracted and uncontracted states to observe corresponding structural changes in the myonemes. We identify the presence of centrin in myoneme fibers via immunogold labeling. We find that the appearance of these fibers changes from a loose network in uncontracted cells to a denser, more well-defined fiber when contracted. These structural changes shed a new perspective on how myonemes may generate force. In the long term, mechanical insight into how nature generates this ultrafast contraction may find applications in both understanding and controlling biological force generation.