Ca2+-dependent nuclear contraction in the heliozoon Actinophrys sol

Ca2+-dependent contractility was found to exist in the nucleus of the heliozoon protozoan Actinophtys sol. Upon addition of Ca2+ ([Ca2+](free) =2.0 x 10(-3) M), diameters of isolated and detergent-extracted nuclei became reduced from 16.5 +/- 1.7 mu m to 11.0 +/- 1.3 mu m. The threshold level of [Ca2+](free) for the nuclear contraction was 2.9 x 10(-7) M. The nuclear contraction was not induced by Mg2+, and was not inhibited by colchicine or cytochalasin B. Contracted nuclei became expanded when Ca2+ was removed by EGTA; thus cycles of contraction and expansion could be repeated many times by alternating addition of Ca2+ and EGTA. The Ca2+-dependent nuclear contractility remained even after high salt treatment, suggesting a possible involvement of nucleoskeletal components in the nuclear contraction. Electron microscopy showed that, in the relaxed state, filamentous structures were observed to spread in the nucleus to form a network. After addition of Ca2+, they became aggregated and constructed a mass of thicker filaments, followed by re-distribution of the filaments spread around inside of the nucleus when Ca2+ was removed. These results suggest that the nuclear contraction is induced by Ca2+-dependent transformation of the filamentous structures in the nucleus. (c) 2005 Elsevier Ltd. All rights reserved.