Nuclear envelope

The nuclear envelope is made up of two lipid bilayer membranes, an inner nuclear membrane and an outer nuclear membrane. These membranes are connected to each other by nuclear pores. Two sets of intermediate filaments provide support for the nuclear envelope. An internal network forms the nuclear lamina on the inner nuclear membrane. A looser network forms outside to give external support. While it is physically linked, the outer nuclear membrane contains proteins found in far higher concentrations than the endoplasmic reticulum. All four nesprin proteins (nuclear envelope spectrin repeat proteins) present in mammals are expressed in the outer nuclear membrane. Nesprin proteins connect cytoskeletal filaments to the nucleoskeleton. Nesprin-mediated connections to the cytoskeleton contribute to nuclear positioning and to the cell's mechanosensory function. KASH domain proteins of Nesprin-1 and -2 are part of a LINC complex (linker of nucleoskeleton and cytoskeleton) and can bind directly to cystoskeletal components, such as actin filaments, or can bind to proteins in the perinuclear space. Nesprin-3 and -4 may play a role in unloading enormous cargo; Nesprin-3 proteins bind plectin and link the nuclear envelope to cytoplasmic intermediate filaments. Nesprin-4 proteins bind the plus end directed motor kinesin-1. The outer nuclear membrane is also involved in development, as it fuses with the inner nuclear membrane to form nuclear pores. Inner membrane The inner nuclear membrane encloses the nucleoplasm, and is covered by the nuclear lamina, a mesh of intermediate filaments which stabilizes the nuclear membrane as well as being involved in chromatin function. It is lined with a fiber network called the nuclear lamina which is 10-40 nm thick and provides strength. Mutations in the genes that encode for the inner nuclear membrane proteins can cause several laminopathies. Nuclear pores The nuclear envelope is punctured by around a thousand nuclear pore complexes, about 100 nm across, with an inner channel about 40 nm wide. The complexes contain a number of nucleoporins, proteins that link the inner and outer nuclear membranes. ==Cell division==

During the G2 phase of interphase, the nuclear membrane increases its surface area and doubles its number of nuclear pore complexes. Electron and fluorescence microscopy has given strong evidence that the nuclear membrane is absorbed by the endoplasmic reticulum—nuclear proteins not normally found in the endoplasmic reticulum show up during mitosis. This transient rupture is likely caused by nuclear deformation. The rupture is rapidly repaired by a process dependent on "endosomal sorting complexes required for transport" (ESCRT) made up of cytosolic protein complexes. Reformation Exactly how the nuclear membrane reforms during telophase of mitosis is debated. Two theories exist— • Vesicle fusion — where vesicles of nuclear membrane fuse together to rebuild the nuclear membrane • Re-shaping of the endoplasmic reticulum—where the parts of the endoplasmic reticulum containing the absorbed nuclear membrane envelop the nuclear space, reforming a closed membrane. ==Origin of the nuclear membrane==

A study of the comparative genomics, evolution and origins of the nuclear membrane led to the proposal that the nucleus emerged in the primitive eukaryotic ancestor (the "prekaryote"), and was triggered by the archaeo-bacterial symbiosis. Several ideas have been proposed for the evolutionary origin of the nuclear membrane. These ideas include the invagination of the plasma membrane in a prokaryote ancestor, or the formation of a genuine new membrane system following the establishment of proto-mitochondria in the archaeal host. The adaptive function of the nuclear membrane may have been to serve as a barrier to protect the genome from reactive oxygen species (ROS) produced by the cells' pre-mitochondria. ==Notes==