What makes the contraction work?

Using electron and immunofluorescence microscopy, the team examined the organism in fine detail. They found that calcium ions initiate the contraction, while a fishnet-like protein structure carries it out.

Single-celled organisms lack muscle fibers. Instead, Spirostomum contains fibrous structures called myonemes, built from two calcium-binding proteins: centrin and Sfi1. In this organism, the myonemes form a web around the outside of the cell. When contraction begins, the network tightens inward, then returns to its original shape.

"The fishnet geometry is unique because it lets Spirostomum contract uniformly, which protects its internal organelles (single cells' versions of organs) while it moves so quickly," Elting says. "It works because the Sfi1 protein in the myoneme can shift from stiff to flexible. In the presence of calcium ions Sfi1 loses its stiffness and clumps up like a ball of wet spaghetti, which causes the fishnet to pull tight, shrinking the organism."

Why is this so different from muscle?

Human muscles run on adenosine triphosphate, or ATP — a molecule that stores and releases chemical energy. Spirostomum appears to rely on a fundamentally different process.

"Comparing the way our muscles contract to the way Spirostomum works is like comparing gas to electric power," Elting says. "ATP undergoes a chemical change and gets 'burned up,' like gasoline, whereas calcium ions act like an electrical current, although we still don't know what produces the voltage that starts the current, or how it gets 'reset' so contraction can happen again."

That reset mechanism remains one of the biggest open questions. Calcium-triggered reactions should, in principle, fire once and stop. Yet Spirostomum contracts repeatedly. The researchers now want to pin down exactly how calcium triggers the contraction and how the organism prepares to do it again.

"We would expect calcium-triggered reactions to be 'one shot,' but Spirostomum can do it repeatedly," Elting says. "Understanding those aspects of its motion are the keys to building a fast-moving, ATP-independent artificial muscle."

What could this mean for engineering?

The findings, while preliminary, offer concrete leads for engineers seeking artificial muscles that move quickly without depending on ATP. If researchers can work out how the organism triggers and resets its calcium-driven system, those principles could be adapted for synthetic devices.

The study's co-corresponding authors are Aaron Dinner of the University of Chicago, Jerry Honts of Drake University, and Saad Bhamla of the University of Colorado Boulder. First author Joseph Lannan, a former Ph.D. student at NC State, led the work, with additional contributions from researchers at NC State, the University of Chicago, Georgia Tech, and UC San Francisco.

The research was supported by the National Science Foundation and by the National Institute of General Medical Sciences of the National Institutes of Health.