Distinct egg membrane vesicles differing in binding and fusion properties contribute to sea urchin male pronuclear envelopes formed in vitro.

We have identified three distinct membrane vesicle populations from sea urchin egg cytoplasm that cooperate in assembling the male pronuclear envelope in vitro. Membranes from sea urchin egg homogenates were separated by buoyant density into five vesicle fractions, three of which bind to demembranated sperm nuclei. Each requires a membranous element (lipophilic structure) derived from the sperm nuclear envelope at the tip and base (poles) of the nucleus in order to bind. Binding is differentially sensitive to protease, high salt and N-ethyl maleimide treatment of the membrane vesicles. MV1 binds at the poles and is required for fusion of the membrane vesicle fractions to each other and to the lipophilic structures. MV2 beta binds over the entire chromatin surface and is enriched in an endoplasmic reticulum marker enzyme. MV2 alpha binds at the nuclear poles, is enriched in a Golgi enzyme marker and is required for fusion of MV2 beta. All three fractions are required for nuclear envelope formation in vitro. The results suggest a multistep process for nuclear envelope formation involving contributions from both sperm and egg, roles for both endoplasmic reticulum and non-endoplasmic reticulum-derived vesicles, and the localization of a critical element of the fusion machinery in MV1.