Cytoskeleton as well as molecular-motor proteins existing in protista cells are responsible for both: generating their movement and maintaining the cell shape. The main players here are cytoskeletal proteins: tubulin and actin, accompanied by molecular motors: dyneins, kinesins and myosins. This system works under the control of numerous regulatory proteins and smaller molecules such as calcium ions and cyclic nucleotides.

The motile system in ciliate protists is based on axoneme, which is the most representative and conservative structure consisting of microtubule and dynein. Extracellular stimuli are converted in these eukaryotic cells through signal transduction mechanisms to generate intracellular second messengers, Ca²⁺ and cyclic nucleotides. These molecular signals, amongst other, control the ciliary locomotor system by modulating the activity of axoneme, changing the direction and frequency of effective ciliary beating. The primary role in regulating the mechanisms of axoneme motility by second messengers is accomplished by phosphorylation and dephosphorylation of axoneme proteins. Ca²⁺ may also influence the levels of cAMP and cGMP by controlling the activity of cAMP and cGMP cyclases. In addition, in regulation of axoneme activity voltage-dependent ion channels (K⁺ and Ca²⁺) are involved, thus affecting the cell membrane potential in these cells.

Quite another mean of movement is used by crawling amoebae, Amoeba proteus, Acanthamoeba castellanii or ameboic fragments of Dictyostelium discoideum. The main players of motility regulation in those cells are calcium ions and RhoA family small G proteins. RhoA regulates activity of effector proteins, including Rho dependent kinase (ROCK). Those effector proteins regulate activity of actomyosin. As in case of tissue cells, actomyosin contraction is regulated in protista in calcium dependent manner, and rise of cytoplasmic calcium leads to the actomyosin contraction. Contrary to tissue cells the different is way of myosin II phosphorylation in regulation of this process. In multicellular organisms, phosphorylation of MLC leads to its sensitization for calcium signal. In A. castellanii, D. discoideum and probably A. proteus it is myosin heavy chain phosphorylation what desensitizies actomyosin for calcium signal. Nevertheless the rest of signaling machinery works in the same way, leading to the reverse answer to known signaling pathways, and situation where inhibition of RhoA/ROCK signaling leads to activation of actomyosin contractility.

• Legal notice:
  

  Copyright (c) Pielaszek Research, all rights reserved.
  The above materials, including auxiliary resources, are subject to Publisher's copyright and the Author(s) intellectual rights. Without limiting Author(s) rights under respective Copyright Transfer Agreement, no part of the above documents may be reproduced without the express written permission of Pielaszek Research, the Publisher. Express permission from the Author(s) is required to use the above materials for academic purposes, such as lectures or scientific presentations.
  In every case, proper references including Author(s) name(s) and URL of this webpage: https://science24.com/paper/10874 must be provided.

1. Phosducin-like protein and chaperonin CCT are associated with Tetrahymena tubulin involved in cilia biogenesis