MODIFICATIONS OF ANIONIC-LIPID DOMAINS PRECEDING MEMBRANE-FUSION IN GUINEA-PIG SPERM

The relationship between anionic-lipid concentration and the functional properties of plasma membrane domains was explored using the guinea pig sperm membrane as a model, with polymyxin B (PXB) as a probe. Areas of plasmalemma specialized for fusion during the acrosome reaction had a higher affinity for the probe than adjacent nonfusigenic regions. Capacitation, a process preceding acrosome-plasma membrane fusion, markedly enlarged the area susceptible to PXB binding over the acrosomal cap. Protease treatment mimicked capacitation by increasing the acrosome-reaction incidence as well as PXB binding, at enzyme concentrations not affecting the surface coat nor altering filipin/sterol localization. Both proteolytic digestion and capacitation failed to augment PXB- or filipin-affinity in nonfusigenic zones, such as the post-acrosomal segment, including its particle-free maculae. Incubation of sperm in capacitating medium supplemented with 32P-labeled phosphate, followed by lipid extraction, TLC and autoradiography, revealed a radioactive band comigrating with cardiolipin and phosphatidic acid. Vermiform protrusions elicited by PXB in the outer lamellae of cardiolipin-phosphatidylcholine liposomes resembled those seen in fusional regions of sperm membrane. At differing concentrations of anionic lipids are found in adjacent domains of the sperm plasma membrane; these domains mirror the functional regions of the membrane, with higher anionic-lipid concentrations localized over fusional zones; the surface coat does not participate in the maintenance of such domains; anionic-lipid concentrations localized over fusional zones; the surface coat does not participate in the maintenance of such domains; anionic-lipid synthesis may contribute to their formation; and anionic-lipid concentrations increase as the membrane becomes fusionally competent, indicating that cellular modulation of lipid domains accompanies regulation of membrane function.