Rearrangements of sea urchin egg cytoplasmic membrane domains at fertilization

Fertilization in the sea urchin is accompanied by rapid reorganization of t he egg endoplasmic reticulum (ER), ER-derived vesicles contribute to one of three classes of membranes used in assembling the male pronuclear envelope in vitro. We provide here biochemical evidence for the rearrangement of se a urchin egg cytoplasmic membrane domains at fertilization up to the first mitosis, with respect to two nuclear envelope markers, lamin B and lamin B receptor (LBR), using purified vesicles prepared from homogenates fractiona ted by floatation on sucrose gradients. In unfertilized eggs, immunoprecipi tation data indicate that most of lamin B and LBR are localized in the same vesicles but do not interact. By 3 min post-fertilization, both proteins a re more widely distributed across the gradients and by 12 min most of lamin B and LBR are localized in vesicles of different densities. This partition ing is maintained throughout S phase, At mitosis, most lamin B and LBR rema in in distinct vesicles, while a small proportion of lamin B and LBR, likel y derived from the disassembled nuclear envelope, associate in a minor subs et of vesicles. The results illustrate a dynamic reorganization of egg cyto plasmic membranes at fertilization, and the establishment of distinct membr ane domains enriched in specific nuclear envelope markers during the first cell cycle of sea urchin development. Additionally, we demonstrate that mal e pronuclear membrane assembly occurs only when both cytosol and membranes originate from fertilized but not unfertilized eggs, suggesting that fertil ization-induced membrane rearrangements contribute to the ability of the eg g to assemble the male pronuclear envelope.