Motility processes in Acantharia. II. A Ca²⁺ dependent system of contractile 2–4 nm filaments isolated from demembranated myonemes

Myonemes of the acantharians are contractile ribbon-like organelles. As previously shown, their motility is based on the coiling mechanism of double-twisted 2–4 nm nonactin filaments [14]. Myonemes have been isolated and manipulated in vitro. After demembranation, the contraction takes place when the Ca²⁺ concentration is above 10⁻⁷ M, whereas relaxation occurs below this threshold concentration. The response to Ca²⁺ ions is an on/off mechanism. Both contraction and relaxation can be induced repeatedly without fatigue phenomena. Other divalent cations such as Sr²⁺, Ba²⁺, Ma²⁺, CO²⁺, and La³⁺ can replace Ca²⁺ in inducing contraction of the demembranated myonemes although with less efficiency. Contraction and relaxation are ATP-independent and calmodulin is not involved in this in vitro motility process. The myoneme is a strongly resistant structure which is capable of contracting and relaxing under various extreme conditions which indicates very stable proteins and resistant functions.