Actin filament translocations in sea urchin eggs

Rhodamine (rh) phalloidin was used as a probe for actin filaments in living eggs and was observed using confocal microscopy. Exogenous, rh phalloidin labeled actin filaments were injected into eggs and were observed to translocate in the cytoplasm. At fertilization, filaments accumulated at the fertilization cone. Rh phalloidin alone, injected at concentrations that did not interfere with fertilization or mitosis, labeled filaments in the interior of unfertilized eggs, some of which translocated. At fertilization, staining increased dramatically at the fertilization cone and to a lesser degree around the entire cortex. At about 9 min, many filaments appeared to detach and translocated away from the egg surface. This study shows that there are mechanisms that can translocate actin filaments within the egg cytoplasm (at approximately 0.2-0.3 mu m/sec), and raises the possibility that at least some of the changes in actin filament organization after fertilization are due to translocation of filaments.