Calcium-dependent regulator protein: Localization in mitotic apparatus of eukaryotic cells

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Abstract
Ca-dependent regulator protein is a low MW (17,000), thermostable, Ca binding protein which is structurally homologous to skeletal muscle troponin C. This protein is present in all nonmuscle cells and appears with stress fibers in interphase cells by indirect immunofluorescence. Using this procedure, the distribution of the protein during mitosis of eukaryotic cells [mouse BALB/c 3T3 cells, rat kangaroo Pt k1 cells] was investigated. As the cells enter prophase, the distinct cytoplasmic localization disappears commensurate with the dissolution of the cytoskeleton. The regulator protein seems to be randomly distributed throughout the prophase cell, including the region around the condensed chromosomes. At prometaphase, it is localized in association with the half-spindles of the mitotic apparatus. Through metaphase and most of anaphase, the protein remains localized between the chromosomes and the poles of the spindle. During late anaphase the protein is also found in the interzone region but rapidly condenses into 2 small regions, one on each side of the midbody that separates the daughter cells. The regulator protein is not localized in the cleavage furrow during telophase, whereas actin is demonstrable in this region. Indeed, placement of the protein during mitosis is distinct from both that of actin and that of tubulin. The localization of Ca-dependent regulator protein during mitosis suggests that it may mediate the Ca effects on the mitotic apparatus and play a role in chromosome movement.