Modulation of calcium sensitivity by a specific cortical protein during sea urchin egg cortical vesicle exocytosis

A study of the Ca2+ sensitivity of cortical vesicle (CV) discharge has been accomplished using isolated sea urchin egg cortices. Cortices isolated in a medium ionically similar to normal egg cytoplasm discharge 50% of their CVs at 1.6 microM Ca2+ (=[Ca2+]50). Alternatively, cortices isolated in a medium containing 500 mM chaotropic anions (Cl-, Br-, I-, or NO-3) discharge their CVs at 16 microM [Ca2+]50. Incubation with the 500 mM KCl extract of cortices restores high Ca2+ sensitivity and the mode of CV discharge characteristic of cortices before extraction. Fractionation of egg homogenates by differential centrifugation reveals that about 20% of the total restoring activity is associated with the cortex. In eggs of Hemicentrotus pulcherrimus, the factor responsible for this restorative function is a heat and protease labile protein with a molecular weight of 100,000. Similar activity is seen also in the eggs and sperm of other species of sea urchin.