Changes in membrane-microfilament interaction in intercellular adherens junctions upon removal of extracellular Ca2+ ions.

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Abstract
EGTA-induced depletion of Ca2+ ions from the culture medium of Madin-Darby bovine kidney epithelial cells results in rapid splitting of adherens-type junctions and the detachment of the vinculin- and actin-containing filament bundle from the cytoplasmic faces of the plasma membrane of the zonula adhaerens. This process was monitored by phase-contrast microscopy, combined with electron microscopy and immunofluorescent localization of the two proteins. It is shown that shortly after extracellular free Ca2+ concentration is lowered to the micromolar range, the actin-containing, junction-associated belt of microfilaments, together with the vinculin-rich junctional plaque material, is irreversibly detached as one structural unit from the plasma membrane, contracts, and is displaced towards the perinuclear cytoplasm where it gradually disintegrates. Other actin- and vinculin-containing structures present in the same cells, notably the focal contacts at the substratum, are not similarly affected by the Ca2+ depletion and retain both the adhesion to the external surface and the association with the plaque and microfilament components. Electron microscopic examination has shown that the membrane domain of the zonulae adhaerentes, unlike that of desmosomes, is not endocytosed after Ca2+ removal and that the displaced actin- and vinculin-containing plaque and filament belt are not associated with a particular membrane. It is further shown that upon restoration of normal Ca2+ levels in the culture medium, new intercellular contacts are established gradually by accretion of both vinculin and actin into new belt-like plaque- and microfilament-containing structures.