Ultrastructural immunocytochemical localization of calmodulin in cultured cells

Using an antibody prepared against performic acid-treated calmodulin, we have localized calmodulin in cultured fibroblastic cells by immunofluorescence and immunoelectron microscopy. In interphase cells, calmodulin was found to be diffusely distributed throughout the cytosol. An increased amount of calmodulin was found in the pericentriolar region of interphase cells. No significant aggregation of calmodulin was found in association with microfilaments, peripheral cytoplasmic microtubules or clathrin-coated structures. Calmodulin was present in moderate amounts in microvilli, ruffles, and zeiotic blebs of the cell surface. In motitic cells, calmodulin was found concentrated in the pericentriolar region, and appeared to concentrate along radiating spindle microtubules proximal to the centrioles. Redistribution of calmodulin was seen between early and late telophase, in which the pericentriolar pattern of calmodulin in early telophase shifted to an aggregation on the intercellular bridge, with a large part of the midbody portion of the bridge being devoid of calmodulin. These results show that calmodulin is distributed throughout the cytosol, but is markedly concentrated in the region of the microtubule organizing center in interphase cells, as well as in elements of the mitotic spindle apparatus. This distribution suggests that calmodulin has a regulatory role in the organization and function of microtubules during interphase, as well as during mitosis.