A Rho-signaling pathway mediates cortical granule translocation in the sea urchin oocyte

Cortical granules are secretory vesicles of the egg that play a fundamental role in preventing polyspermy at fertilization. In the sea urchin egg, they localize directly beneath the plasma membrane forming a compact monolayer and, upon fertilization, undergo a Ca²⁺-dependent exocytosis. Cortical granules form during early oogenesis and, during maturation, translocate from the cytosol to the oocyte cortex in a microfilament-mediated process. We tested the hypothesis that these cortical granule dynamics were regulated by Rho, a GTPase of the Ras superfamily. We observed that Rho is synthesized early in oogenesis, mainly in a soluble form. At the end of maturation, however, Rho associates with cortical granules. Inhibition of Rho with the C3 transferase from C. botulinum blocks cortical granule translocation and microfilaments undergo a significant disorganization. A similar effect is observed by GGTI-286, a geranylgeranyl transferase inhibitor, suggesting that the association of Rho with the cortical granules is indispensable for its function. In contrast, the anchorage of the cortical granules in the cortex, as well as their fusion at fertilization, are Rho-independent processes. We conclude that Rho association with the cortical granules is a critical regulatory step in their translocation to the egg cortex.