Abstract
We have investigated the formation of the male pronuclear envelope in a cell-free system consisting of permeabilized sea urchin sperm nuclei incubated in fertilized sea urchin egg cytosolic extract together with purified cytoplasmic membrane vesicles. In fertilized egg cytosol, the sperm nuclear lamina disassembles as a result of lamin B phosphorylation mediated by egg protein kinase C. The conical sperm nucleus decondenses into a spherical pronucleus in a cytosol- and ATP-dependent manner. Assembly of the pronuclear envelope is a lamin-independent process involving ATP-dependent binding of vesicles to chromatin and GTP-dependent fusion of vesicles to each other and to specialized polar lipophilic structures of the sperm nucleus. Three egg cytoplasmic membrane vesicle fractions with distinct biochemical, chromatin-binding and fusion properties, are required for pronuclear envelope assembly. Targeting of the bulk of nuclear envelope vesicles to chromatin is mediated by a lamin B receptor (LBR)-like integral membrane protein. During the last step of pronuclear formation involving nuclear swelling, lamin B is imported into the nucleus from a soluble pool, and physically associates with LBR. Our results indicate that male pronuclear envelope assembly in vitro involves a highly ordered series of reactions.
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