We used the acrosome reaction of boar sperm cells to study the dynamics of surface transmembrane glycoproteins (TMG) during secretion. The acrosome reaction is the Ca2+-dependent fusion of a large cytoplasmic vesicle (the acrosome) with the overlaying segment of the plasma membrane (acrosomal cap) that leads to the release of the acrosomal enzymes. We followed the acrosome reaction by freeze-fracture electron microscopy, in particular the topographical rearrangement of a population of acrosomal-cap intramembrane particles that we showed before to correspond to transmembrane lectin (wheat germ agglutinin)-binding proteins (Aguas & Pinto da Silva, J. Cell. Biol., 97:1356). We found that these TMG move in the direction of either one of two opposite poles, proximal and distal, of the cap. This bimodal movement of the TMG reorganizes the acrosomal cap into three domains: the first two, the apical rim and the equator, are membrane domains where the integral proteins are directed to, accumulated, and form small aggregates; the third, comprises the extensive in-between area from where TMG were displaced. The topography of the new membrane domains of the acrosomal cap is now coincident with that of the unaltered domains of the acrosomal membrane underneath. The mirroring of the acrosomal membrane by the plasma membrane is followed by fusion between the two membranes, formation of a labyrinth of tubules, and exocytosis of the acrosomal contents through intertubular fenestrae.
