ORGANELLE MARKERS - Nucleus

LAP1, LAP2

In eukaryotic cells, the nuclear envelope consists of inner and outer membranes which define the border between the nucleus and cytoplasm.
The inner and outer nuclear membranes are joined by large, supramolecular assemblies called nuclear pore complexes through which various molecules pass into and out of the nucleus. The inner membrane is lined by a filamentous meshwork called the nuclear lamina made up of proteins called lamins. The attachment of the lamins to the inner nuclear membrane appears to occur through their association with 4 related integral membrane proteins called Lamin Associated Polypeptides (LAPs). LAP 1A, 1B & 1C share a high degree of sequence homology while LAP 2 is distinct from the others.
LAP 1A & B bind lamins A, C and B1, while LAP 2 binds lamin B1 only.
In addition to lamin binding, LAP 2 has been shown to bind to chromosomes. Phosphorylation of LAP 2 during mitosis inhibits binding to both lamins and chromosomes suggesting that LAP 2 plays an important role in nuclear envelope re-assembly.

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O-linked Glycoproteins

Diffusion of metabolites and small non-nuclear molecules as well as active, mediated import of protein and export of protein and RNA through the nuclear envelope occurs through nuclear pore complexes or NPC’s. NPC’s contain up to 100 different polypeptides which have a combined mass of about 125 megadaltons. The channel available for passive transport through the NPC is about 9-10 nm in diameter while carrier mediated changes in the NPC result in a ~25 nm channel used for larger, actively transported molecules. Of the 100 polypeptides, at least 8 of these are O-linked N-acetylglycosamine-modified in mammalian cells. All of the mammalian O-linked glycoproteins contain multiple copies of phenylalanine, glycine dipeptide repeats dispersed throughout part of their sequence. Studies indicate that the NPC O-linked glycoproteins have a direct role in nuclear protein import. The accumulation of proteins in the nucleus is mediated by short sequences of basic amino acids called nuclear localization sequences (NLSs). These sequences are necessary and sufficient to direct a protein or inert carrier to the nuclear interior. Nuclear protein import is accomplished by two sequential, energy dependent events. The first step, docking at the nuclear pore complex, requires a 54/56 kDa nuclear localization signal receptor (a-karyopherin, importin-a, SRP-a) and the nuclear transport factor p97 (NTF97, b-karyopherin, importin-b). The second step, translocation across the nuclear envelope (NE), requires the GTP-binding protein, Ran/TC4.

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c-jun

c-jun is the cellular homolog of the avian sarcoma virus 17 transforming protein, v-jun. It is a 39 kDa (predicted molecular mass) transcriptional activator that is part of the transcriptional complex called AP1 and is a member of the b-ZIP family of proteins. c-jun forms homo/heterodimers with Jun, Fos, ATF, or other trancriptional factors through its leucine zipper domain. Transcription by c-jun is induced by cellular exposure to growth factors, cytokines, UV light and other DNA damaging agents. Activity of c-jun is activated by phosphorylation of serine 63 and serine 73 residues by JNK (c-jun N-terminal kinase). Recent studies have shown that c-jun plays an important role in neuronal apoptosis, regeneration and protection.

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Estrogen Receptors

The Estrogen Receptors (ER), are members of the nuclear steroid hormone receptor family and are also found in the plasma membrane. Steroid hormone receptors are composed of three domains, an N-terminal domain required for maximal transactivation function (TAF), a DNA binding domain and a steroid binding domain. The DNA binding domain is separated from the steroid binding domain by a hinge region. The DNA binding domain has a high degree of homology amongst the various steroid hormone receptors, followed by the steroid binding domain. The N-terminus is the most divergent region among members of the steroid receptor family.

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Histone H1

Histone H1 binds DNA between core histone complexes of histones H2A, H2B, H3 and H4. It is a basic protein that serves as a convenient substrate for a wide range of protein kinases including protein kinase C, cdc2 kinase, cyclin-dependent kinase 5 and cAMPdependent protein kinase.

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Nuclear factor-kappa B

Nuclear factor-kappa B, also known as NFkappaB, KBF1, p110 and EBP-1, was identified as a sequence specific transcriptional activator that binds to the intronic enhancer of kappa light chain gene in B lymphocytes (1,2). NF-?B is a heterodimer that consists of a 50 kDa DNA binding subunit (p50) and a 65 kDa transactivation subunit (p65/Rel A). Both of these subunits exhibit sequence homology to the protooncogene c-Rel (4). The p50 has an isoform called p49/p52, and both proteins are derived from the amino-terminal of precursor protein p105 and p100, respectively. The p50/p65 heterodimer remains in the cytosol in an inactive form as a complex with its inhibitor, I?B. Upon stimulation of cells by a wide variety of stimuli such as lipopolysaccharide (LPS), pro-inflammatory cytokines (interleukin-1, tumor necrosis factor), and viral infection, I?B is phosphorylated and degraded by proteosome. The active NF- ?B heterodimer is translocated into the nucleus and will induce gene expression.

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Poly (ADP-ribose) polymerase (PARP)

Poly (ADP-ribose) polymerase (PARP) is a 113kDa nuclear protein that is involved in DNA repair and is an early marker of apoptosis. PARP is activated by DNA strand breaks to produce poly (ADPribose) polymers and attach them to nuclear proteins such as histone, topoisomerase, DNA-dependent Protein kinase,and itself. PARP contains an amino terminal zinc-finger containing-DNA binding domain, an automodification domain and a carboxyl-terminal catalytic domain. Upon induction of apoptosis in HL-60 cells treated with etoposide, PARP is cleaved by caspase between Asp 214 and Gly-215 into an 85kDa and a 29kDa fragment. PARP-knock out mice display no phenotypic abnormalities, and apoptosed normally in response to TNF, ?-radiation, dexamethasone, suggesting PARP is dispensable for apoptosis.

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NTF97

The accumulation of proteins in the nucleus is mediated by short sequences of basic amino acids called nuclear localization sequences (NLSs). These sequences are necessary and sufficient to direct a protein or inert carrier to the nuclear interior. Nuclear protein import is accomplished by two sequential, energy dependent events. The first step, docking at the nuclear pore complex, requires a 54/56 kDa nuclear localization signal receptor (a-karyopherin, importin-a, SRP-a) and the nuclear transport factor p97 (NTF 97, b-karyopherin, importin-b). The second step, translocation across the nuclear envelope (NE), requires the GTP-binding protein, Ran/TC4.

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Fibrillarin

Nop1p / Fibrillarin was originally identified as a nucleolar protein of bakers yeast, Saccharomyces cerevisiae (accession P15646). The Nop1p protein is essential for yeast viability and is localized in the nucleoli. The human homologue of Nop1p is fibrillarin (accession P22087) a component of the nucleolar small nuclear ribonucleoprotein (snRNP) particle. The human fibrillarin gene is located on chromosome 19 (19q13.1). Fibrillarin proteins have been cloned and sequenced from several other species (Mouse, accession P35550, Xenopus accession P22232, C. elegans accession Q22053, and S. pombe accession P35551). The N terminal 80 amino acids contain multiple copies based on the peptide RGG, and the remaining 240 amino acids consist of the fibrillarin domain. A fibrillarin homologue has also been identified in the genome of the archean Methanococcus (accession NC_000909). This protein lacks the RGG rich N-terminal extension but is clearly homologues to the other sequences throughout all of the fibrillarin domain. The structure of this molecule has been determined and shown to consist of 2 extended b-sheets flanked by 4 a-helixes. Patients with the autoimmune disease scleroderma often have strong circulating autoantibodies to a 34kDa protein which was subsequently found to be fibrillarin. Fibrillarin is an excellent marker for the nucleolus.

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