Abstract:
The reconstitution of a replication-competent, transcriptionally activenucleus following mitosis, fertilization or nuclear transplantation involves astepwise series of reactions, most (if not all) of which are controlled by thecytoplasmic environment. This review discusses the nature of cytoplasmiccontributions to the development of the male pronucleus at fertilization, andthe effect of altering the cytoplasmic environment on nuclear assembly. Thesystem used to investigate these regulations consists of permeabilized seaurchin sperm nuclei incubated under controlled conditions in a cell-freeextract of fertilized sea urchin eggs. (1) In eggcytoplasmic extract, male pronuclear formation is initiated by the disassemblyof the sperm nuclear lamina as a result of lamin phosphorylation by acytosolic protein kinase C. (2) Sperm histones arephosphorylated by an as yet unidentified soluble kinase.(3) The conical sperm nucleus decondenses into aspherical pronucleus in an ATP-and cytosolic pH-dependent manner.(4) Chromatin decondensation is associated with thereplacement of sperm histones by maternal histones. (5)Nuclear membranes form by ATP-dependent binding of vesicles to chromatin andGTP-dependent fusion of these vesicles to one another.(6) Three cytoplasmic vesicle populations with distinctbiochemical, chromatin-binding and fusion properties are required for nuclearenvelope assembly. (7) Targeting of the bulk of nuclearmembrane vesicles to chromatin is mediated by an integral membrane proteinsimilar to human lamin B receptor. (8) The last step ofmale pronuclear formation, nuclear swelling, is promoted by the assembly ofnuclear pores, nuclear import of soluble lamins and growth of the nuclearmembranes. (9) Once inside the nucleus, lamin Bassociates with lamin B receptors, presumably to tether the inner nuclearmembrane with the lamina. Overall, these processes are similar to thosecharacterizing nuclear reconstitution after mitosis in somatic cells ornuclear remodeling following transplantation into oocytes or eggs. Theinfluence of the egg cytoplasmic environment on some aspects of nuclearremodeling after nuclear transplantation is also discussed.