A sea urchin cell-free system to study male pronuclear assembly and activation

Typically sperm nuclei are genetically inert and contain extremely compacted chromatin. Following fertilization, the first steps in their conversion to somatic nuclei (male pronuclei) which will support further development involve chromatin decondensation and the formation of a new nuclear envelope. We have studied the reactivation of sea urchin sperm nuclei in a cell-free system derived from homogenates of activated sea urchin egg cytoplasm. The cell-free system has provided several novel insights including requirements for sperm-specific histone phosphorylation on N- and C-terminal extensions and disassembly of the sperm nuclear lamina for decondensation, the utilization of remnant regions of the sperm nuclear envelope to direct polarized binding and fusion of egg membranes to form the new nuclear envelope, and a role for phosphoinositide metabolism in initiation of membrane fusion through binding of a minor membrane fraction enriched in PtdIns (4,5)P₂, PLCγ and SFK1 which locally produces a fusigenic lipid, diacylglycerol.