Cytoplasmic control of nuclear assembly.

The reconstitution of a replication-competent, transcriptionally active nucleus following mitosis, fertilization or nuclear transplantation involves a stepwise series of reactions, most (if not all) of which are controlled by the cytoplasmic environment. This review discusses the nature of cytoplasmic contributions to the development of the male pronucleus at fertilization, and the effect of altering the cytoplasmic environment on nuclear assembly. The system used to investigate these regulations consists of permeabilized sea urchin sperm nuclei incubated under controlled conditions in a cell-free extract of fertilized sea urchin eggs. (1) In egg cytoplasmic extract, male pronuclear formation is initiated by the disassembly of the sperm nuclear lamina as a result of lamin phosphorylation by a cytosolic protein kinase C. (2) Sperm histones are phosphorylated by an as yet unidentified soluble kinase. (3) The conical sperm nucleus decondenses into a spherical pronucleus in an ATP-and cytosolic pH-dependent manner. (4) Chromatin decondensation is associated with the replacement of sperm histones by maternal histones. (5) Nuclear membranes form by ATP-dependent binding of vesicles to chromatin and GTP-dependent fusion of these vesicles to one another. (6) Three cytoplasmic vesicle populations with distinct biochemical, chromatin-binding and fusion properties are required for nuclear envelope assembly. (7) Targeting of the bulk of nuclear membrane vesicles to chromatin is mediated by an integral membrane protein similar to human lamin B receptor. (8) The last step of male pronuclear formation, nuclear swelling, is promoted by the assembly of nuclear pores, nuclear import of soluble lamins and growth of the nuclear membranes. (9) Once inside the nucleus, lamin B associates with lamin B receptors, presumably to tether the inner nuclear membrane with the lamina. Overall, these processes are similar to those characterizing nuclear reconstitution after mitosis in somatic cells or nuclear remodeling following transplantation into oocytes or eggs. The influence of the egg cytoplasmic environment on some aspects of nuclear remodeling after nuclear transplantation is also discussed.