An analysis of the lamellar structure of sea urchin egg cortical granules using X-ray scattering

Cortical granules (CGs) are secretory vesicles associated with egg and oocy te plasma membranes that undergo exocytosis at fertilisation. In the sea ur chin Strongylocentrotus purpuratus, the internal organisation of these CGs exhibits a lamellar-type morphology. The different lamellar lavers correspo nd to proteoglycans, structural proteins and enzymes required for fertilisa tion envelope assembly and modification of the post-fertilisation egg surfa ce. We have studied the lamellar structure of CGs using X-ray scattering an d reveal the contrast density variation of the lamellae in the native state . The structure of functionally competent CGs in situ differs significantly from that determined by electron microscopic studies. We observed a strong periodicity of the lamellar structure of 280 Angstrom as opposed to the 59 0 Angstrom repeat observed previously. Fusion of the CGs produced a loss of the lamellar repeat and the development of a broad peak corresponding to a 20 Angstrom periodicity that may be indicative of the molecular packing in the resulting hydrated gel structure. (C) 2000 Federation of European Bioc hemical Societies. Published by Elsevier Science B.V. All rights reserved.