Nucleoskeleton

Editor-In-Chief: Henry A. Hoff

The nucleoskeleton (NSK) provides a framework for DNA replication, transcription, chromatin remodeling, signaling, and mRNA synthesis, processing and transport.

Introduction
Of the structures local to the nucleoplasm, some serve to confine it such as the inner membrane of the nuclear envelope. While others are completely suspended within it, for example, the nucleolus.

Other structural elements include the nuclear lamina, core filaments, diffuse skeleton, nuclear bodies, the intranucleolar skeleton and fibrillar centers.

Nuclear envelope membrane skeleton
Emerin 18 kDa (no NLS) mediates inner nuclear membrane anchorage to the nuclear lamina, regulates the flux of beta-catenin into the nucleus, and interacts with nuclear actin.

Nesprin is a protein that is found in the outer nuclear membrane. It attaches to the actin filaments of the cytoskeleton and to emerin which is found in the inner nuclear membrane. Nesprin-1 is an isoform of enaptin which associates with the F-actin cytoskeleton and the nuclear membrane. Nesprin-2, like nesprin-1, is associated with actin-binding and nuclear envelope associated proteins. Nesprin-2 is an important scaffold protein implicated in the maintenance of nuclear envelope architecture.

Nesprins have KASH [an acronym for Klarsicht, Anc-1, Syne Homology] domains which are conserved C-terminal protein regions of less than ~30 amino acids. KASH domains always follow a transmembrane domain and SUN [Sad1 and UNc (uncoordinated)-84 homology] domain proteins. In mammals, three KASH-domain proteins are represented, termed nesprin-1, -2 and -3. The C-terminal of the KASH domain contains a single transmembrane segment, followed by an evolutionarily conserved sequence (9–32 amino acids), located in the PNS (perinuclear space). The KASH domain is necessary and sufficient for nuclear envelope (NE) targeting.

Most proteins containing KASH domains may be involved in the positioning of the nucleus in the cell. These domains may interact with proteins containing SUN domains in the space between the outer and inner nuclear membranes to bridge the nuclear envelope and transfer force from the nucleoskeleton to the cytoskeleton. Although KASH proteins probably localize to the outer nuclear membrane, some KASH protein isoforms may localize to the inner nuclear membrane.

Like proteins containing KASH domains most proteins containing SUN domains may be involved in the positioning of the nucleus in the cell. SUN domains may interact directly with KASH domains in the space between the outer and inner nuclear membranes to bridge the nuclear envelope and transfer force from the nucleoskeleton to the cytoskeleton. SUN proteins may preferentially localize to the inner nuclear membrane.

Nuclear lamina
The nuclear lamina is a dense, ~ 30 to 100 nanometers thick, fibrillar network composed of intermediate filaments made of lamin that lines the inner surface of the nuclear envelope in animal cells.

The nuclear pore complexes are embedded in the nuclear lamina.

Nuclear pore complex
The nuclear pore complex restricts the size of particles including molecules that can get into the nucleoplasm to be incorporated into the nucleoskeleton. Larger proteins require a nuclear localization signal (NLS). The pores are 100 nm in total diameter, with an opening diameter of about 50 nm; however, the gap through which molecules freely diffuse is only about 9-10 nm wide, due to the presence of regulatory systems within the center of the pore. The 10 nm diameter corresponds to an upper mass limit of 70 kDa.

Due to the size limitation of the nuclear pore, these polypeptides would range from 9 kDa to <70 kDa and not need or have a NLS.

The lamins of mammalian nuclei are polypeptides of 60-80 kDa: A (70 kDa), B (68 kDa), and C (60 kDa). A- and B-type lamins, which form separate, but interacting, stable meshworks in the lamina, have different mobilities. All of the lamins have a NLS.

Actin nucleoskeleton
Actins are highly conserved proteins that can form a tetramer. Further polymerization produces microfilaments such as those of the cytoskeleton.

Nuclear actins
Alpha actin: ACTA1 42 kDa is myosin binding and forms actin microfilaments, ACTA2 42 kDa is at least intracellular. As a part of nuclear actin, beta-actin (ACTB) has a 42 kDa mass as a monomer (no NLS), is a component of SWI/SNF chromatin remodeling complexes, and rapidly shuttles between the nucleosol and cytosol. Gamma actin: ACTC1 42 kDa is intracellular, incorporated into actin microfilaments, is dissolved in the cytosol.

Nuclear actin-related proteins (ARPs)
The actin-related proteins (Arps) are also components of these chromatin remodeling complexes. ACTR1A 42.6 kDa is a subunit of dynactin, as is ACTR1B 42.3 kDa. Both subunits are Arp1 (centractin). Arp2 45 kDa is a major constituent of the Arp2/3 complex. Arp3 47 kDa is a major constituent of the Arp2/3 complex which binds actin and may initiate nucleus assembly of actin. Arp5 68 kDa has no NLS but is involved in ATP-dependent chromatin remodeling. Arp6 20 kDa and 36 kDa forms has nuclear localization. Arp8 58 kDa, 37 kDa and 70 kDa chromosome associates with involvement in chromatin remodelling (has NLS). Arp10 46 kDa. ArpM1 41 kDa. The Arp2/3 complex may be involved in the control of actin polymerization. ARPC1A 41 kDa is a subunit of the Arp2/3 complex along with ARPC1B 41 kDa, ARPC2 34 kDa, ARPC3 21 kDa, ARPC3B 21 kDa, ARPC4 20 kDa, ARPC5 16 kDa, and ARPC5L 17 kDa.

Nuclear actin-like proteins (ACTLs)
Actin-like 6B (ACTL6B) is an Arp, a subunit of the BAF complex, which is functionally related to SWI/SNF complex, 47 kDa, and has intracellular nucleus localization. As with ACTL6B, ACTL6A 53 kDa and 43 kDa (two different isoforms) binds chromatin, localizes to the nucleus, is involved in chromatin remodeling, and transcription. It is a structural constituent of the nucleoskeleton. ACTRT1 42 kDa. ACTRT2 42 kDa.

Nuclear actin binding proteins (ABPs)
Supervillin (SVIL) has NLSs contained in its amino-terminus. The carboxy-terminus contains numerous consecutive sequences with extensive similarity to proteins in the gelsolin family of actin-binding proteins, which cap, nucleate, and/or sever actin filaments. SVIL is tightly associated with both actin filaments and plasma membranes, suggesting a role as a high-affinity link between the actin cytoskeleton and the membrane. Its function may include recruitment of actin and other cytoskeletal proteins into specialized structures at the plasma membrane and in the nuclei of growing cells.

SVIL is known to localize to chromosomes. It also bundles F-actin. And, binds myosin II which is not in the nucleus.

One motor protein that localizes intracellularly to the nucleus is myosin 1F 125 kDa. It does not have a NLS. Its N terminal motor domain uses ATP and has actin binding sites. The tail of MYO1F includes (1) a TH-1 region which may interact with membrane phospholipids, (2) a TH-2 proline-rich region which may contain an ATP-insensitive actin-binding site, and (3) a SH-3 domain found in a variety of cytoskeletal and signaling proteins.

Nuclear matrix
Still others such as the nuclear matrix are found throughout the inside of the nucleus.

Nucleoplasmic veil
Lamins within the nucleoplasm form another regular structure the nucleoplasmic veil. The veil is excluded from the nucleolus and is present during interphase. The lamin structures that make up the veil bind chromatin and disrupting their structure inhibits transcription of protein-coding genes. Changes also occur in the lamina mesh size.

Intranuclear structures
In addition to forming the nuclear lamina, the lamins can form intranuclear structures.

Microtubular structures
Dynactin 1 (Dctn1) 150 kDa, the largest subunit of dynactin, which binds microtubules and is involved in chromosome movement, interacts with dynein. And, it has a NLS, is microtubule plus-end binding, has a second isoform of 135 kDa , and binds to Arp1. Dynactin 2 (dynamitin) 50 kDa, 4-5 copies per dynactin molecule, is intracellular to the nucleus. Dynactin 3 22 kDa is a part of the dynactin complex, binding directly to Dctn1, and is intracellular to the nucleus. Dynactin 4 (Dctn4) 52 kDa is intracellular to the nucleus and a member of the pointed-end complex which contains Dctn4-6 and ACTR10. Dynactin 5 20 kDa and dynactin 6 21 kDa can easily localize to the nucleus.

The most common members of the tubulin family are α-tubulin ~55 kDa and β-tubulin ~55 kDa, both of which make up microtubules. Microtubules are assembled from heterodimers of one α- and one β-tubulin. β-tubulin has GTPase activity; i.e., hydrolyzes GTP to GDP whereas α-tubulin does not. Katanin is a heteromeric microtubule-severing protein. It contains a 60 kDa ATPase subunit, which functions to sever microtubules. The second 80 kDa subunit regulates the activity of the ATPase and localizes the protein to the centrosomes. Katanin forms 14–16 nm rings in its active oligomerized state on the walls of microtubules. Even though katanin is brought into the nucleus, it does not act only in mitosis or meiosis. Katanin-mediated severing may serve to maintain organization by promoting microtubule disassembly, regulating their length for efficient movement, and releasing microtubules from the centrosome.

Nucleolar skeleton
Nucleolar skeletons are observable with light and electron microscopy and are characterized by ravels of filaments that are especially densely packed in the nucleolar cortex.

DNA as well as RNA are not constituents of this structure.

This insoluble protein filament complex may form a skeleton specific to the nucleolus proper that is different from other extraction-resistant components of the nucleus such as the matrix and lamina and is involved in the spatial organization of the nucleolar chromatin and its transcriptional products. It is a framework of nucleolar filaments. The nucleolar skeletons are roughly spheroidal like the nucleolus itself and constitute the nucleolar cortex. The interior is formed by relatively loosely packed tangles of filaments ~4 nm thick arranged in higher-order coils of 30-40 nm dia. In the periphery are aggregates of various sizes representing local ravels of filament packing, with many filament continuities to internal filament elements.

The skeleton contains considerable amounts of nuclear actin. The fibrillar meshworks indistinguishable from the nucleolar skeleton structure are also observable in anucleolate mutants, forming so-called “pseudo-nucleoli” suggesting that such skeletal proteins can assemble into spheroidal meshwork structures independent of the presence of nucleolar chromatin.

Acknowledgements
The content on this page was first contributed by: Henry A. Hoff.

Initial content for this page in some instances came from Wikipedia.