In the blink of an eye: Modeling structural transformations during the ultrafast contraction of a single-celled organism

Spriostomum ambiguum is a ciliated protist that has one of the fastest contraction rates observed in all of biology, reaching speeds around 200 body lengths per second.  This rapid contraction is mediated by an influx of Ca²⁺, which binds to protein filaments called myonemes that comprise a mesh-like structure near the plasma membrane.  The binding of Ca²⁺ causes the individual filaments in the mesh to suddenly shorten, so that the entire organism contracts.  An exciting possibility is that this Ca²⁺-powered contractile machinery can serve as a controllable artificial cytoskeletal structure for sub-cellular force generation.  However, the mechanical forces that control the changes in the microstructure of the myoneme mesh during contraction are not well understood.  In this talk, we will discuss light microscopy measurements that allow for detailed quantification of the myoneme mesh structure.  These measurements guided the development of a computational model for the mechanics of the mesh, in which myoneme filaments are spatially resolved and the relevant constraints on the mesh are included.  The computational model reproduces the observed configurations before and after contraction, and it reveals the competing mechanical forces that balance in the equilibrated states.