Filipin/sterol complexes in fertilized and unfertilized sea urchin egg membranes

Sea urchin (Arbacia punctulata) eggs and zygotes were treated with filipin in an effort to examine changes in membrane sterols at fertilization. The plasma membrane of treated unfertilized eggs possessed numerous filipin/sterol complexes, while fewer complexes were associated with membranes delimiting cortical granules, demonstrating that the plasmalemma is relatively rich in beta-hydroxysterols in comparison to cortical granule membrane. Following fusion with the plasmalemma, membrane formerly delimiting cortical granules underwent a dramatic alteration in sterol composition, as indicated by a rapid increase in the number of filipin/sterol complexes. In contrast, portions of the zygote plasma membrane, derived from the plasmalemma of the unfertilized egg, displayed little or no change in filipin/sterol composition. Other than regions of the plasma membrane engaged in endocytosis, the plasmalemma of the zygote possessed a homogeneous distribution of filipin/sterol complexes and appeared similar to that of the unfertilized egg. These results demonstrate that following its fusion with the egg plasmalemma, membranes, formerly delimiting cortical granules, undergo a dramatic alteration in sterol composition. Changes in the localization of filipin/sterol complexes are discussed in reference to alterations in egg plasmalemmal function at fertilization.