Accumulation of fluorescently labeled actin in the cortical layer in sea urchin eggs after fertilization

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Abstract
Actin from sea urchin eggs was fluorescently labeled with fluorescein isothiocyanate (FITC), N‐(7‐dimethylamino‐4‐methylcoumarinyl)‐maleimide (DACM), or 5‐iodoacetamidofluorescein (IAF) and microinjected into sea urchin eggs and oocytes. It distributed evenly in the cytoplasm of unfertilized eggs. Upon fertilization, actin accumulated first around the sperm binding site and, soon afterwards, in the fertilization cone. The accumulation propagated all over the cortex after a latent period of 10–20 sec. In the case of Clypeaster japonicus eggs, propagation of the accumulation coincided with a shape change in the egg, suggesting that the accumulated actin in the cortex generates forces. FITC‐actin was incorporated into microvilli and retained in the cortex after cleavage. On the other hand, DACM‐ or IAF‐actin was not incorporated into microvilli and was dispersed from the cortex by cleavage. These differences may be attributable to differences in the properties of the actins labeled at different sites. After photobleaching by laser light irradiation, FITC‐ or IAF‐actin redistributed in the cortex of fertilized egg as quickly as it did before fertilization. When an unfertilized egg was injected with both actin and a calcium buffer (intracellular free Ca2+ concentration 9 μM), the actin accumulation was similar to that during fertilization but without the latent period. This suggests that the accumulation depended on the increase in the intracellular free Ca2+ concentration. When the unfertilized egg was injected with 0.2 M EGTA after injection of labeled actin and then inseminated, it accumulated only in the protrusion of cytoplasm where the sperm had entered, and fertilization was not completed. In immature oocytes, the accumulation was observed in the cortical region, including the huge protrusion of the cytoplasm where the sperm had entered. These results suggest that actin accumulation in the sperm binding site plays an important role in the sperm reception mechanism of the egg.