![]() Ĭholesterol has several functions in natural membranes, including acting as a scaffold for targeting various proteins. Cholesterol confers resistance to detergent solubilization and increases phospholipid packing. DRMs obtained in this manner are enriched in cholesterol. DRMs are commonly isolated from whole cell lysates by sucrose gradient centrifugation and purified using specific protein markers such as glycosyl-phosphoinositide (GPI)-anchored proteins –. DRMs have been subject to a plethora of investigations and the current paradigm predicts that DRMs are typically enriched in cholesterol and saturated phospholipids. Initially the only physical property attributed to these regions was that they are by definition detergent-resistant membranes (DRMs). ![]() It is important to note that the available evidence suggests these detergent-resistant membranous regions are not created by the extraction procedures since they are visible in untreated sperm, and both in vitro and in vivo studies have shown that the nuclear envelope remnants become incorporated into the newly formed nuclear envelope,. We refer to these detergent treated sperm nuclei as 0.1% nuclei. ![]() To mimic in vivo envelope disassembly, sperm nuclei were extracted with a 0.1% non-ionic detergent so that the lateral nuclear envelope was removed and the only membranous material left was at the two poles. Thus the remnants are likely to have a role both in binding of precursor membranes and initiation of membrane fusion. From these studies three major points have emerged: 1) without the envelope remnants the egg nuclear membrane precursor vesicles do not bind to the nucleus and thus nuclear envelope formation is prevented, 2) the binding of the egg nuclear membrane precursor vesicles proceeds progressively from the sites of the remnants in the acrosomal and centriolar fossae, eventually surrounding the entire surface, and 3) one subfraction of egg vesicles (MV1) is required for the remnants to be incorporated into the new nuclear envelope and can fuse with the remnants without the other vesicles. Their function has mainly been studied in cell free preparations from fertilized sea urchin egg cytoplasm using a well-established system for nuclear envelope assembly on exogenously added sperm –. Similar remnant membranes are conserved from annelid to mammalian sperm. Egg membranes bind to this nucleus and then fuse to form a nuclear envelope with pores, incorporating the polar sperm nuclear envelope remnants into the new male pronuclear envelope. During this process, remnants of the sperm nuclear envelope at the tip and base of the nucleus, which line two cup-shaped cavities (the acrosomal and centriolar fossae), are not disassembled. Most of this envelope is rapidly disassembled, its membranes vesiculating as the sperm chromatin decondenses from its compact conoïd shape to a uniformly euchromatic spherical mass. At fertilization, the sea urchin egg is activated by the sperm whose nucleus enters with an envelope devoid of pores.
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