POLAR LOCALIZATION OF A DIHYDROPYRIDINE RECEPTOR ON LIVING FUCUS ZYGOTES

We have used a fluorescently-labeled dihydropyridine (FL-DHP) to vital ly stain living Fucus zygotes during the establishment of cell polarit y. Localization of FL-DHP is primarily at the plasma membrane and FL-D HP binding is competitively blocked by an unlabeled dihydropyridine. D istribution of FL-DHP is initially symmetrical before fixation of the polar axis, but becomes asymmetrical in response to a unilateral light gradient. The distribution of FL-DHP receptors can be relocalized whe n the direction of the photopolarizing stimulus is changed. Treatment of cells with cytochalasin B prior to axis fixation reversibly prevent s localization of FL-DHP receptors. Observation of FL-DHP labeling by time-lapse fluorescence microscopy indicates that the existing recepto rs are redistributed during polar axis formation. The asymmetric distr ibution of FL-DHP receptors coincides temporally and spatially with in creased local intracellular calcium ion concentrations, as measured by calcium green dextran. Based on the site, timing, photo-reversibility , and actin dependence of the asymmetric localization of FL-DHP recept or, we conclude that FL-DHP is a vital probe for the later stage of po lar axis formation in Fucus zygotes. Furthermore, we propose that FL-D HP receptors correspond to ion channels that are transported to the fu ture site of polar growth to create the changes in local calcium conce ntration required for polarity establishment.