Motility mechanisms in the actinopods (Protozoa): A review with particular attention to axopodial contraction/extension, and movement of nonactin filament systems

Abstract

AbstractSeveral motility phenomena displayed by members of the Heliozoa, Radiolaria, and Acantharia (Protozoa, Actinopoda) are reviewed. These phenomena include (1) cytoplasmic streaming, internal saltatory motion, and transport of particles at the cell surface; (2) axopod contraction and extension; and (3) contraction of nonactin filament systems.Cytoplasmic streaming and saltatory motion require energy derived from oxidative phosphorylation. In addition, calcium appears to be involved in the regulation of these movements, and a role for calmodulin is suggested. At present, the molecular basis for these motility phenomena remains obscure.We have focused our attention on the rapid contraction of axopods and stalk in the marine heliozoan Actinocoryne contractilis (Febvre‐Chevalier: J. Marine Biol. Assoc. U.K. 60:901‐928, 1980). Contraction is accompanied by the cataclysmic breakdown of microtubules. For this species, a Na+ and Ca2+‐dependent action potential precedes axopod contraction. A lack of contraction in Ca2+‐free media (10−7 M Ca2+) suggests that Ca2+ fluxes across the cell membrane are required.Motile phenomena associated with nonactin filaments of fibrous systems in actinopods, especially in the myonemes of the acantharians (Febvre and Febvre‐Chevalier: Biol. Cell. 44:283–304, 1982) are also examined. In vitro contraction of these filaments is CA2+ dependent and ATP independent' cycles of contraction and extension are caused by Ca2+‐dependent conformational changes in pairs of twisted filaments. In vivo, the Ca2+ dependent contraction of these myonemes may be independent of direct mitochondrial control, but metabolic ATP and calmodulin may be required to regulate the level of free cytoplasmic calcium.