CHARACTERIZATION OF MOESIN IN THE SEA-URCHIN LYTECHINUS-VARIEGATUS REDISTRIBUTION TO THE PLASMA-MEMBRANE FOLLOWING FERTILIZATION IS INHIBITED BY CYTOCHALASIN-B

We have investigated the distribution and function of an ezrin-radixin -moesin-like (ERM) molecule in the sea urchin. A sea urchin homologue of moesin was cloned that shares 75% amino acid similarity in the cons erved N-terminal region to other moesin molecules. A 6.3 kb message is transcribed late in embryogenesis and is present in adult tissues. Po lyclonal antibodies were generated to proteins expressed by a bacteria l expression vector, and affinity purified. These antibodies recognize a single 75 kDa protein that is present throughout development in app roximately equal abundance, and specifically they immunoprecipitate a single protein. We show by immunolocalization that SUmoesin has two pr edominant patterns during development. First, SUmoesin is rapidly redi stributed after fertilization from a location throughout the egg cytop lasm to a location in the egg cortex. Later in embryogenesis, SUmoesin is localized to the apical ends of cells in the regions of cell-cell junctions. We show that SUmoesin is present in actin-rich regions of t he embryo. Finally, we show that the location of SUmoesin requires an intact actin-based cytoskeleton. SUmoesin fails to localize to the pla sma membrane after fertilization in the presence of cytochalasin B. Fu rthermore, SUmoesin loses its apical position in the region of cell-ce ll junctions in the presence of cytochalasin B in later stages of embr yogenesis. This effect is reversible, and the microtubule inhibitor co lchicine has no effect. These results show that SUmoesin becomes assoc iated with apical plasma membrane structures early in development, and that SUmoesin is both coincident with actin and requires the assembly of actin filmaments to maintain its localization.