The nuclear lamina is a dense (~30 to 100 nm thick) fibrillar network inside the nucleus of eukaryote cells. It is composed of intermediate filaments and membrane associated proteins. Besides providing mechanical support, the nuclear lamina regulates important cellular events such as DNA replication and cell division. Additionally, it participates in chromatin organization and it anchors the nuclear pore complexes embedded in the nuclear envelope.
The nuclear lamina is associated with the inner face of the inner nuclear membrane of the nuclear envelope, whereas the outer face of the outer nuclear membrane is continuous with the endoplasmic reticulum. The nuclear lamina is similar in structure to the nuclear matrix, that extends throughout the nucleoplasm.
The nuclear lamina consists of two components, lamins and nuclear lamin-associated membrane proteins. The lamins are type V intermediate filaments which can be categorized as either A-type (lamin A, C) or B-type (lamin B1, B2) according to homology of their DNA sequences, biochemical properties and cellular localization during the cell cycle. Type V intermediate filaments differ from cytoplasmic intermediate filaments in the way that they have an extended rod domain (42 amino acid longer), that they all carry a nuclear localization signal (NLS) at their C-terminus and that they display typical tertiary structures. Lamin polypeptides have an almost complete α-helical conformation with multiple α-helical domains separated by non-α-helical linkers that are highly conserved in length and amino acid sequence. Both the C-terminus and the N-terminus are non α-helical, with the C-terminus displaying a globular structure with immunoglobulin type folded motif. Their molecular weight ranges from 60 to 80 kilodaltons (kDa).
In the amino acid sequence of a nuclear lamin, there are also two phosphoacceptor sites present, flanking the central rod domain. A phosphorylation event at the onset of mitosis leads to a conformational change which causes the disassembly of the nuclear lamina.
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The nuclear envelope, also known as the nuclear membrane, is made up of two lipid bilayer membranes that in eukaryotic cells surround the nucleus, which encloses the genetic material. The nuclear envelope consists of two lipid bilayer membranes: an inner nuclear membrane and an outer nuclear membrane. The space between the membranes is called the perinuclear space. It is usually about 10–50 nm wide. The outer nuclear membrane is continuous with the endoplasmic reticulum membrane.
The nucleoplasm, also known as karyoplasm, is the type of protoplasm that makes up the cell nucleus, the most prominent organelle of the eukaryotic cell. It is enclosed by the nuclear envelope, also known as the nuclear membrane. The nucleoplasm resembles the cytoplasm of a eukaryotic cell in that it is a gel-like substance found within a membrane, although the nucleoplasm only fills out the space in the nucleus and has its own unique functions.
Intermediate filaments (IFs) are cytoskeletal structural components found in the cells of vertebrates, and many invertebrates. Homologues of the IF protein have been noted in an invertebrate, the cephalochordate Branchiostoma. Intermediate filaments are composed of a family of related proteins sharing common structural and sequence features. Initially designated 'intermediate' because their average diameter (10 nm) is between those of narrower microfilaments (actin) and wider myosin filaments found in muscle cells, the diameter of intermediate filaments is now commonly compared to actin microfilaments (7 nm) and microtubules (25 nm).
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