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Abstract. We have identified an integral membrane protein of sea urchin gametes with an apparent molecular mass of 56 kD that cross-reacts with an antibody against the nucleoplasmic NH2-terminal domain of human lamin B receptor (LBR). In mature sperm, p56 is located at the tip and base of the nucleus from where it is removed by egg cytosol in vitro. In the egg, p56 is present in a subset of cytoplasmic membranes (MV2f3) which contributes the bulk of the nuclear envelope during male pronuclear formation, p56-containing vesicles are required for nuclear envelope assembly and have a chromatin-binding capacity that is mediated by p56. Lamin B is not present in these vesicles and is imported into the nucleus from a soluble pool at a later stage of pronuclear formation. Lamin B incorporation and addition of new membranes are necessary for pronuclear swelling and nuclear envelope growth. We suggest that p56 is a sea urchin LBR homologue that targets membranes to chromatin and later anchors the membrane to the lamina.

Membrane fusion underlies many cellular events, including secretion, exocytosis, endocytosis, organelle reconstitution, transport from endoplasmic reticulum to Golgi and nuclear envelope formation. A large number of investigations into membrane fusion indicate various roles for individual members of the phosphoinositide class of membrane lipids. We first review the phosphoinositides as membrane recognition sites and their regulatory functions in membrane fusion. We then consider how modulation of phosphoinositides and their products may affect the structure and dynamics of natural membranes facilitating fusion. These diverse roles underscore the importance of these phospholipids in the fusion of biological membranes.

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Disassembly of the sperm nuclear envelope at fertilization is one of the earliest events in the development of the male pronucleus. We report that nuclear lamina disassembly in interphase sea urchin egg cytosol is a result of lamin B phosphorylation mediated by protein kinase C (PKC). Lamin B of permeabilized sea urchin sperm nuclei incubated in fertilized egg G 1 phase cytosolic extract is phosphorylated within 1 min of incubation and solubilized prior to sperm chromatin decondensation. Phosphorylation is Ca 2؉ -dependent. It is reversibly inhibited by the PKC-specific inhibitor chelerythrine, a PKC pseudosubstrate inhibitor peptide, and a PKC substrate peptide, but not by inhibitors of PKA, p34 cdc2 or calmodulin kinase II. Phosphorylation is inhibited by immunodepletion of cytosolic PKC and restored by addition of purified rat brain PKC. Sperm lamin B is a substrate for rat brain PKC in vitro, resulting in lamin B solubilization. Two-dimensional phosphopeptide maps of lamin B phosphorylated by the cytosolic kinase and by purified rat PKC are virtually identical. These data suggest that PKC is the major kinase required for interphase disassembly of the sperm lamina.The nuclear lamina consists of a polymeric network of intermediate filament molecules, the nuclear lamins, underlying the inner nuclear membrane. The lamina is a dynamic structure, undergoing expansion during interphase of the cell cycle, and depolymerization at mitosis upon breakdown of the nuclear envelope (NE) 1 (1). Mitotic disassembly and reassembly of the lamina is regulated by reversible lamin phosphorylation and dephosphorylation (1). Interphase lamin phosphorylation has also been reported (2-6), but its significance is not fully understood.Several lamin kinases have been identified that promote mitotic lamina solubilization or inhibit lamina assembly in vitro. They include cyclin B/p34 cdc2 (7), S6 kinase II (8), protein kinase C (PKC) (4, 9), and the cAMP-dependent protein kinase PKA (10). Down-regulation of PKA has also been shown to be essential for mitotic lamina disassembly (11). Although not a lamin kinase, Ca 2ϩ /calmodulin-dependent kinase II (CaM kinase II) is also involved in mitotic NE breakdown in sea urchin embryos (12). PKC has also been shown to phosphorylate chicken lamin B 2 in interphase, a process thought to regulate lamin import into the nucleus (13). These observations imply that multiple kinases regulate the dynamics of the nuclear lamina during the cell cycle. The transformation of the sea urchin sperm nucleus into a pronucleus at fertilization provides an opportunity to investigate NE assembly/disassembly during interphase. Sea urchin eggs are fertilized in G 1 phase of the first cell cycle after completion of both meiotic divisions. At fertilization, the sperm NE vesiculates and a new NE reforms around the male pronucleus as the sperm chromatin decondenses (14). Male pronuclear formation has been duplicated in a cell-free system by incubating detergent-permeabilized sperm nuclei in fertilized egg extracts (15)(16)(1...