Sulfation of fucoidin in Fucus embryos : IV. Autoradiographic investigations of fucoidin sulfation and secretion during differentiation and the effect of cytochalasin treatment

Abstract

Following short pulses with Na235SO4, embryos ofFucus were prepared for autoradiography to determine the cellular distribution of sulfating sites for the sulfated polysaccharide fucoidin. Fucoidin was sulfated throughout the cell at both the rhizoid and the thallus poles when 18-hr zygotes were pulsed with35S. If zygotes were chased for 30 min or more, the [35S]fucoidin appeared to be transported via Golgi vesicles and was secreted into the entire wall with greatest localization at the rhizoid tip. In two-celled embryos, however, sulfation occurred primarily in the rhizoid cell. Although the number of Golgi bodies appeared to be equally distributed near the poles of these two cells, the number in the perinuclear region of the rhizoid cell was greater than that found in the thallus perinuclear region. When observed with the electron microscope, Golgi bodies in the thallus cell appeared to be inactive as evidenced by an absence of hypertrophied cisternae and associated vesicles, which were typical of Golgi bodies in the rhizoid cell. Growth in seawater containing methionine prevented sulfation but did not interfere with the morphological differentiation of the rhizoid and thallus cells. Unlike normal embryos, both cells contained Golgi bodies that appeared to be inactive. Short pulses of35S under these conditions showed that sulfating sites (i.e., Golgi bodies) were distributed throughout the cytoplasm and in the perinuclear regions of both cells of the two-celled embryo. [35S]Fucoidin was secreted into the wall surrounding both thallus and rhizoid cells in two-celled embryos grown with methionine. Cytochalasins B and D at 50 μg/ml reversibly inhibited photopolarization and polar axis fixation but not fucoidin sulfation. Directional transport of Golgi vesicles to the rhizoid was prevented when zygotes were treated with the cytochalasins. These results suggest that sulfation of fucoidin and a cytochalasin-sensitive process are required for localization. The relation of these conclusions to recent proposals for axis fixation and vesicle movement in the fucoid embryo is discussed.