Protein and lipid signaling in membrane fusion: nuclear envelope assembly

Membrane fusion is important in many cell processes including membrane trafficking, mitotic reconstitution of organelles, viral infection and fertilization. Several fusion events occur just prior and subsequent to fertilization in the sea urchin, including the sperm acrosomal reaction, fusion of sperm and egg plasma membranes, exocytosis of cortical granules, reassembly of the sperm nuclear envelope and fusion of the male and female pronuclear envelopes leading to the zygote nucleus of the one-cell embryo. The study of male pronuclear membrane dynamics with cell-free extracts of fertilized sea urchin eggs has revealed several novel features, in particular a structural role arising from altering phospholipids prior to nuclear membrane formation. Fusion of chromatin-bound membrane vesicles in vitro can be triggered by either GTP hydrolysis or exogenous phosphatidyl inositol phospholipase C (PI-PLC). Recent data strongly implicate a role for diacylglycerol in nuclear envelope formation as a structural destabilizing lipid in membrane fusion. Moreover, the endogenous enzyme, PI-PLCγ, is >100-fold enriched in a nuclear envelope precursor vesicle population (MV1) that is required for nuclear envelope assembly. NMR and mass spectrometry analyses show that MV1 contains high levels of phosphoinositides, including the substrate of PLCγ, compared to the other nuclear envelope precursor membranes. MV1 exists in eggs as vesicles in the cortex distinct from the endoplasmic reticulum which contributes most of the nuclear envelope membrane. PLCγ is activated by a tyrosine kinase in response to GTP hydrolysis at an early stage of nuclear envelope formation suggesting a role in initiation of fusion and revealing aspects of a signaling mechanism leading to fusion. The binding of MV1 to two poles of the sperm nucleus offers spatial as well as temporal control of the initiation phase.