Identification, isolation, and properties of a plasma membrane protein involved in the adhesion of boar sperm to the porcine zona pellucida

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Abstract
Boar sperm plasma membranes contain an integral protein (Mr 55 kDa) that apparently functions in the adhesion of sperm to the zona pellucida (Peterson and Hunt: J Cell Biol 105:170a, 1987.) In experiments described in this report, the protein is identified after additional steps of purification involving lectin affinity chromatography and preparative PAGE. An active form of the adhesion protein (APz) develops or becomes first exposed in the corpus epididymis and is fully active in the cauda epididymis; a significant portion of this conformationally labile protein, while integral to the plasma membrane, cannot be solubilized by nonionic detergents and may be associated with the membrane skeleton. APz does not exhibit enzymatic properties thought possibly to be involved in sperm-zona interaction in this and other species. Galactosyltransferase substrates and inhibitors and anliproteases including soybean trypsin inhibitor, pepstatin, leupeptin, and p-aminoben-zamidine failed to block sperm from binding to porcine eggs. Boar sperm proacrosin and antiproacrosin antibody failed to inhibit sperm-egg binding. When plasma membranes or fractions containing APz that bind to dextran sulfate agarose were chromatographed on L-fucose agarose, a sugar which binds proacrosin, plasma membrane proteins that bound to the column failed to absorb anti-APz antibody, Anti-APz was absorbed by fractions that did not contain proacrosin. These data indicate that APz is not proacrosin. Since anti-APz monovalcnt antibody raised from whole cauda or corpus sperm plasma membranes or from chromatographic fractions containing APz completely block capacitated sperm from binding to eggs, and since the ability of this antibody to be absorbed develops as sperm become capable of binding to eggs, we view AP, to be the major and perhaps only plasma membrane protein involved in the adhesion of capacitated boar sperm to eggs prior to the acrosome reaction.

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