Subcellular localization of the GTP-binding protein Rho in the sea urchin sperm

The Rho proteins are small G-proteins that belong to the Ras superfamily and play an essential role in the organization of the actin cytoskeleton. They are characteristically ADP-ribosylated by the exoenzyme C3 from Clostridium botulinum. Sea urchin sperm contain multiple small G proteins (28-24 kDa) whose identity and function are unknown. Here, we examined whether some of these proteins corresponded to the Rho subfamily. A sperm homogenate incubated with C3 and [32P]NAD revealed, by electrophoresis and autoradiography, a single radiolabeled band with a molecular mass of 25 kDa; this size was identical, under the same conditions, to that displayed by RhoA from human platelets. In flagellar fractions, the 25 kDa protein ADP-ribosylated by C3 localized in the cytosol and in the plasma membrane. In the sperm head, the 25 kDa protein was detected in a membrane preparation enriched in acrosomal and plasma membranes. Separation of these head membranes through a continuous density gradient revealed that both the 25 kDa protein [32P]ADP-ribosylated by C3 and actin had the same localization as bindin, the adhesive protein characteristic of the acrosome. An antibody against RhoB cross-reacted by immunoblotting with the 25 kDa protein and it revealed by both immunofluorescence and immunogold the presence of Rho in the acrosomal region, the middle piece of the head, and in the flagellum. Thus, the results indicate that the G-protein of 25 kDa previously detected in sea urchin sperm is Rho, likely the type B. Based on its cellular localization, Rho may participate in regulating motility and the actin polymerization that accompanies the acrosome reaction.