The activity of a Ca2+-adenosinetriphosphatase (ATP phosphohydrolase, EC 3.6.1.3) increases during the mitotic phase of synchronized mouse mastocytoma P-815X2 cells. The enzyme is synthesized mainly during a distinct period of the interphase and is activated at mitosis. It is thought to regulate the formation of the mitotic apparatus by controlling the concentration of Ca2+ ions at the site of formation of the mitotic spindle.Ca2+ ions have been implicated as regulating factors in mitosis (1). They can interfere with microtubule assembly (2-4) and a system capable of controlling the concentration of these ions at the place of spindle formation has been tentatively identified (1,5).In sea urchins a Ca2+-adenosinetriphosphatase (ATPase; ATP phosphohydrolase, EC 3.6.1.3) exists which shows in early development cyclic fluctuations during the cell cycle; one peak of activity occurs in the first half of the cell cycle and another one at the time of mitosis (6, 7). Whereas the meaning of the first peak is as yet unclear, some evidence has accumulated that the activity increase of the Ca2+-ATPase at mitosis is involved in the regulation of mitosis. If the length of the cell cycle is altered experimentally, the increase of the enzymatic activity still remains linked with mitosis (7,8). In parthenogenetically activated eggs the activity of the enzyme rises whenever a spindle-like figure is formed (5). If only parts of the cell cycle are turned on, which in sea urchins can be performed by NH3 treatment of unfertilized eggs (9, 10), the Ca2+-ATPase cycle is also turned on, reaching its maximum of activity when the chromosomes show a mitosis-like configuration (11).These and the following results suggest that the enzyme is indeed involved in the regulation of mitosis. Therefore, we propose to name this enzyme the "mitotic Ca2+-ATPase."The experiments so far described were carried out on sea urchin eggs, in which cell divisions follow each other without cell growth during interphases. There, fluctuations of an enzyme activity are likely to represent the activation of pre-existing proteins rather than production of additional enzyme molecules. The same Ca2+-ATPase has also been found in growing cells (mouse fibroblasts) and a cyclic change of activity with a peak at mitosis has been observed (12). The purified enzyme promotes the polymerization of brain tubulin onto isolated mitotic apparatus from L cells (12). The interaction of tubulin from nonneuronal cells with isolated spindles has been reported to be enhanced in the presence of the enzyme (13). In the present work we measure the synthesis and the activity of the mitotic Ca2+-ATPase through the entire cell cycle in synchronized mouse mastocytoma cells. The synthesis of new enzyme proteins takes place during interphase. The peak of the enzymatic activity occurs during mitosis, as in sea urchin, eggs, and must reflect the activation of preexisting enzyme molecules. 1610MATERIAL AND METHODS Cells. Mouse mastocytoma P-815X2 cells (14) were grown in medium I supple...

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The occurrence of the mitotic Ca2+-ATPase, resembling the enzyme described for higher organisms, is demonstrated in multinuclear plasmodia of the myxomycete Physarum polycephalum. The activity of this enzyme undergoes cyclic fluctuations during the synchronous nuclear cycle with a minimum in early G2-phase and a maximum around the time of mitosis.

The mitotic Ca2+-ATPase was isolated from root tips of Vicia faba. The enzyme shows the same characteristics as the one isolated from a variety of animal cells. The results support the hypothesis that the membrane-bound mitotic Ca2+-ATPase is part of a Ca2+-regulating system universal for both plant and animal cells.