Nuclear Envelope Formation In Vitro: A Sea Urchin Egg Cell-Free System

The formation of the nuclear envelope (NE) typically occurs once during every mitotic cycle in somatic cells, and also around the sperm nucleus following fertilization. Much of our understanding of NE assembly has been derived from systems modeling the latter event in vitro. In these systems, demembranated sperm nuclei are combined with fertilized egg cytoplasmic extracts and an ATP-regenerating system and in a multistep process they form the functional double bilayer of the NE. Using a system that we developed from sea urchin gametes, we have demonstrated that NE assembly is regulated by membrane vesicles in a spatial and temporal fashion, emphasizing the roles of phosphoinositides, particularly phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P₂ ), diacylglycerols (DAG), and lipid-modifying enzymes in NE assembly.