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p14ARF (also called ARF tumor suppressor, ARF, p14ARF) is an alternate reading frame protein product of the ''
CDKN2A CDKN2A, also known as cyclin-dependent kinase inhibitor 2A, is a gene which in humans is located at chromosome 9, band p21.3. It is ubiquitously expressed in many tissues and cell types. The gene codes for two proteins, including the INK4 family ...
'' locus (i.e. ''INK4a''/''ARF'' locus). p14ARF is induced in response to elevated
mitogenic A mitogen is a small bioactive protein or peptide that induces a cell to begin cell division, or enhances the rate of division (mitosis). Mitogenesis is the induction (triggering) of mitosis, typically via a mitogen. The mechanism of action of a ...
stimulation, such as aberrant growth signaling from
MYC ''Myc'' is a family of regulator genes and proto-oncogenes that code for transcription factors. The ''Myc'' family consists of three related human genes: ''c-myc'' (MYC), ''l-myc'' ( MYCL), and ''n-myc'' (MYCN). ''c-myc'' (also sometimes refe ...
and
Ras (protein) Ras, from "Rat sarcoma virus", is a family of related proteins that are expressed in all animal cell lineages and organs. All Ras protein family members belong to a class of protein called small GTPase, and are involved in transmitting signals ...
. It accumulates mainly in the
nucleolus The nucleolus (, plural: nucleoli ) is the largest structure in the nucleus of eukaryotic cells. It is best known as the site of ribosome biogenesis, which is the synthesis of ribosomes. The nucleolus also participates in the formation of sig ...
where it forms stable complexes with NPM or Mdm2. These interactions allow p14ARF to act as a
tumor suppressor A tumor suppressor gene (TSG), or anti-oncogene, is a gene that regulates a cell during cell division and replication. If the cell grows uncontrollably, it will result in cancer. When a tumor suppressor gene is mutated, it results in a loss or red ...
by inhibiting
ribosome Ribosomes ( ) are macromolecular machines, found within all cells, that perform biological protein synthesis (mRNA translation). Ribosomes link amino acids together in the order specified by the codons of messenger RNA (mRNA) molecules to ...
biogenesis or initiating
p53 p53, also known as Tumor protein P53, cellular tumor antigen p53 (UniProt name), or transformation-related protein 53 (TRP53) is a regulatory protein that is often mutated in human cancers. The p53 proteins (originally thought to be, and often s ...
-dependent
cell cycle The cell cycle, or cell-division cycle, is the series of events that take place in a cell that cause it to divide into two daughter cells. These events include the duplication of its DNA (DNA replication) and some of its organelles, and subs ...
arrest and
apoptosis Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes (morphology) and death. These changes incl ...
, respectively. p14ARF is an atypical protein, in terms of its transcription, its amino acid composition, and its degradation: it is transcribed in an alternate
reading frame In molecular biology, a reading frame is a way of dividing the nucleic acid sequence, sequence of nucleotides in a nucleic acid (DNA or RNA) molecule into a set of consecutive, non-overlapping triplets. Where these triplets equate to amino acids or ...
of a different protein, it is highly basic, and it is polyubiquinated at the
N-terminus The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the ami ...
. Both
p16INK4a p16 (also known as p16INK4a, cyclin-dependent kinase inhibitor 2A, CDKN2A, multiple tumor suppressor 1 and numerous other synonyms), is a protein that slows cell division by slowing the progression of the cell cycle from the G1 phase to the S p ...
and p14ARF are involved in
cell cycle The cell cycle, or cell-division cycle, is the series of events that take place in a cell that cause it to divide into two daughter cells. These events include the duplication of its DNA (DNA replication) and some of its organelles, and subs ...
regulation. p14ARF inhibits mdm2, thus promoting
p53 p53, also known as Tumor protein P53, cellular tumor antigen p53 (UniProt name), or transformation-related protein 53 (TRP53) is a regulatory protein that is often mutated in human cancers. The p53 proteins (originally thought to be, and often s ...
, which promotes
p21 p21Cip1 (alternatively p21Waf1), also known as cyclin-dependent kinase inhibitor 1 or CDK-interacting protein 1, is a cyclin-dependent kinase inhibitor (CKI) that is capable of inhibiting all cyclin/CDK complexes, though is primarily associated ...
activation, which then binds and inactivates certain
cyclin Cyclin is a family of proteins that controls the progression of a cell through the cell cycle by activating cyclin-dependent kinase (CDK) enzymes or group of enzymes required for synthesis of cell cycle. Etymology Cyclins were originally disco ...
- CDK complexes, which would otherwise promote
transcription Transcription refers to the process of converting sounds (voice, music etc.) into letters or musical notes, or producing a copy of something in another medium, including: Genetics * Transcription (biology), the copying of DNA into RNA, the fir ...
of
gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
s that would carry the
cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...
through the G1/S checkpoint of the cell cycle. Loss of p14ARF by a homozygous mutation in the ''CDKN2A'' (''INK4A'') gene will lead to elevated levels in mdm2 and, therefore, loss of
p53 p53, also known as Tumor protein P53, cellular tumor antigen p53 (UniProt name), or transformation-related protein 53 (TRP53) is a regulatory protein that is often mutated in human cancers. The p53 proteins (originally thought to be, and often s ...
function and cell cycle control. The equivalent in mice is p19ARF.


Background

The p14ARF transcript was first identified in humans in 1995, and its
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
product confirmed in mice that same year. Its gene locus is on the short arm of
chromosome 9 Chromosome 9 is one of the 23 pairs of chromosomes in humans. Humans normally have two copies of this chromosome, as they normally do with all chromosomes. Chromosome 9 spans about 138 million base pairs of nucleic acids (the building blocks of D ...
in humans, and on a corresponding location on chromosome 4 in mice. It is located near the
genes In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
for the tandem repeats INK4a and INK4b, which are 16 kDa (p16INK4a) and 15 kDa (p15INK4b) proteins, respectively. These INK4 proteins directly inhibit the cyclin D-dependent kinases
CDK4 Cyclin-dependent kinase 4 also known as cell division protein kinase 4 is an enzyme that in humans is encoded by the ''CDK4'' gene. CDK4 is a member of the cyclin-dependent kinase family. Function The protein encoded by this gene is a member o ...
and
CDK6 Cell division protein kinase 6 (CDK6) is an enzyme encoded by the ''CDK6'' gene. It is regulated by cyclins, more specifically by Cyclin D proteins and Cyclin-dependent kinase inhibitor proteins. The protein encoded by this gene is a member of th ...
. There are other INK4 genes on other chromosomes, however these are not linked to
cancer Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...
, and so their functions are not likely to be overlapping. An important cyclin-dependent substrate is the
retinoblastoma protein The retinoblastoma protein (protein name abbreviated pRb; gene name abbreviated ''Rb'', ''RB'' or ''RB1'') is a proto-oncogenic tumor suppressor protein that is dysfunctional in several major cancers. One function of pRb is to prevent excessive ...
Rb, which is
phosphorylated In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, whi ...
in late gap 1 phase (
G1 phase The G1 phase, gap 1 phase, or growth 1 phase, is the first of four phases of the cell cycle that takes place in eukaryotic cell division. In this part of interphase, the cell synthesizes mRNA and proteins in preparation for subsequent steps leadi ...
), allowing G1 exit. The Rb protein limits
cell proliferation Cell proliferation is the process by which ''a cell grows and divides to produce two daughter cells''. Cell proliferation leads to an exponential increase in cell number and is therefore a rapid mechanism of tissue growth. Cell proliferation re ...
by blocking the activity of E2F transcription factors, which activate the transcription of genes needed for DNA replication. When Rb is phosphorylated by cyclin D and E-dependent kinases during the G1 phase of the cell cycle, Rb can not block E2F-dependent transcription, and the cell can progress to the DNA synthetic phase(
S phase S phase (Synthesis Phase) is the phase of the cell cycle in which DNA is replicated, occurring between G1 phase and G2 phase. Since accurate duplication of the genome is critical to successful cell division, the processes that occur during ...
). Therefore, INK4a and INK4b serve as tumor suppressors by restricting proliferation though the inhibition of the CDKs responsible for Rb
phosphorylation In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, wh ...
. In addition to the INK4a protein, the unrelated protein, ARF, is transcribed from an alternate
reading frame In molecular biology, a reading frame is a way of dividing the nucleic acid sequence, sequence of nucleotides in a nucleic acid (DNA or RNA) molecule into a set of consecutive, non-overlapping triplets. Where these triplets equate to amino acids or ...
at the INK4a/ARF locus. INK4a and p14ARF mRNA each consist of three
exons An exon is any part of a gene that will form a part of the final mature RNA produced by that gene after introns have been removed by RNA splicing. The term ''exon'' refers to both the DNA sequence within a gene and to the corresponding sequence ...
. They share exons 2 and 3, but there are two different exon 1 transcripts, α and β. Exon 1β (E1β) is intercalated between the genes for INK4a and INK4b. Although exon 1α (E1α) and E1β are about the same in terms of content and size, the 5’ AUG (
start codon The start codon is the first codon of a messenger RNA (mRNA) transcript translated by a ribosome. The start codon always codes for methionine in eukaryotes and Archaea and a N-formylmethionine (fMet) in bacteria, mitochondria and plastids. The ...
) of exon 1β has its own promoter and opens an alternative reading frame in exon 2, hence the name p14ARF (ARF exon 3 is not translated). Because of this, INK4a and p14ARF have unrelated
amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
sequences despite overlapping coding regions and have distinct functions. This dual-use of coding sequences is not commonly seen in mammals, making p14ARF an unusual protein. When the ARF β-transcript was found, it was thought that it probably would not encode a protein. In humans, ARF is translated into the 14kDa, 132 amino acid p14ARF protein, and in mice, it is translated into the 19kDa, 169 amino acid p19Arf. The E1β protein segment of mouse and human ARF are 45% identical, with an overall ARF identity of 50%, compared to a 72% identity between mouse and human INK4a E1α segment, and a 65% overall identity. Although the INK4a and ARF proteins are structurally and functionally different, they are both involved in
cell cycle The cell cycle, or cell-division cycle, is the series of events that take place in a cell that cause it to divide into two daughter cells. These events include the duplication of its DNA (DNA replication) and some of its organelles, and subs ...
progression. Together, their broad inhibitory role may help counter
oncogenic Carcinogenesis, also called oncogenesis or tumorigenesis, is the formation of a cancer, whereby normal cells are transformed into cancer cells. The process is characterized by changes at the cellular, genetic, and epigenetic levels and abno ...
signals. As mentioned above, INK4a inhibits proliferation by indirectly allowing Rb to remain associated with E2F transcription factors. ARF is involved in
p53 p53, also known as Tumor protein P53, cellular tumor antigen p53 (UniProt name), or transformation-related protein 53 (TRP53) is a regulatory protein that is often mutated in human cancers. The p53 proteins (originally thought to be, and often s ...
activation by inhibiting Mdm2 (HDM2 in humans). Mdm2 binds to p53, inhibiting its transcriptional activity. Mdm2 also has E3
ubiquitin Ubiquitin is a small (8.6 kDa) regulatory protein found in most tissues of eukaryotic organisms, i.e., it is found ''ubiquitously''. It was discovered in 1975 by Gideon Goldstein and further characterized throughout the late 1970s and 1980s. Fo ...
ligase activity toward p53, and promotes its exportation from the
cell nucleus The cell nucleus (pl. nuclei; from Latin or , meaning ''kernel'' or ''seed'') is a membrane-bound organelle found in eukaryotic cells. Eukaryotic cells usually have a single nucleus, but a few cell types, such as mammalian red blood cells, h ...
to the
cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. The ...
for degradation. By antagonizing Mdm2, ARF permits the transcriptional activity of p53 that would lead to cell cycle arrest or
apoptosis Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes (morphology) and death. These changes incl ...
. A loss of ARF or p53, therefore, would give cells a survival advantage. The function of ARF has primarily been attributed to its Mdm2/p53 mechanism. ARF does, however, also inhibit proliferation in cells lacking p53 or p53 and Mdm2. In 2004 has been found that one of ARF's p53-independent functions involves its binding to nucleophosmin/B23 (NPM). NPM is an acidic
ribosomal Ribosomes ( ) are macromolecular machines, found within all cells, that perform biological protein synthesis (mRNA translation). Ribosomes link amino acids together in the order specified by the codons of messenger RNA (mRNA) molecules to fo ...
chaperone (protein) In molecular biology, molecular chaperones are proteins that assist the conformational folding or unfolding of large proteins or macromolecular protein complexes. There are a number of classes of molecular chaperones, all of which function to assi ...
involved in preribosomal processing and nuclear exportation independent of p53, and oligomerizes with itself and p14ARF. Nearly half of p14ARF is found in NPM-containing complexes with high molecular mass (2 to 5 MDa). Enforced expression of ARF retards early 47S/45S
rRNA Ribosomal ribonucleic acid (rRNA) is a type of non-coding RNA which is the primary component of ribosomes, essential to all cells. rRNA is a ribozyme which carries out protein synthesis in ribosomes. Ribosomal RNA is transcribed from ribosoma ...
precursor processing and inhibits 32S rRNA cleavage. This suggests that p14ARF can bind to NPM, inhibiting rRNA processing. ARF-null cells have increased nucleolar area, increased
ribosome Ribosomes ( ) are macromolecular machines, found within all cells, that perform biological protein synthesis (mRNA translation). Ribosomes link amino acids together in the order specified by the codons of messenger RNA (mRNA) molecules to ...
biogenesis, and a corresponding increase in
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
synthesis. The larger size resulting from more ribosomes and protein is not associated with increased proliferation, however, and this ARF-null phenotype occurs even though the normal basal levels of Arf are usually low. Knocking down ARF with
siRNA Small interfering RNA (siRNA), sometimes known as short interfering RNA or silencing RNA, is a class of double-stranded RNA at first non-coding RNA molecules, typically 20-24 (normally 21) base pairs in length, similar to miRNA, and operating wi ...
to
exon An exon is any part of a gene that will form a part of the final mature RNA produced by that gene after introns have been removed by RNA splicing. The term ''exon'' refers to both the DNA sequence within a gene and to the corresponding sequen ...
1β results in increased rRNA transcripts, rRNA processing, and ribosome nuclear export. The unrestrained ribosome biogenesis seen when NPM is not bound to ARF does not occur if NPM is also absent. Although the induction of ARF in response to
oncogenic Carcinogenesis, also called oncogenesis or tumorigenesis, is the formation of a cancer, whereby normal cells are transformed into cancer cells. The process is characterized by changes at the cellular, genetic, and epigenetic levels and abno ...
signals is considered to be of primary importance, the low levels of ARF seen in
interphase Interphase is the portion of the cell cycle that is not accompanied by visible changes under the microscope, and includes the G1, S and G2 phases. During interphase, the cell grows (G1), replicates its DNA (S) and prepares for mitosis (G2). A c ...
cells also has a considerable effect in terms of keeping cell growth in check. Therefore, the function of basal level ARF in the NPM/ARF complex appears to be to monitor steady-state ribosome biogenesis and growth independently of preventing proliferation.


Role in Disease

Very commonly, cancer is associated with a loss of function of INK4a, ARF, Rb, or
p53 p53, also known as Tumor protein P53, cellular tumor antigen p53 (UniProt name), or transformation-related protein 53 (TRP53) is a regulatory protein that is often mutated in human cancers. The p53 proteins (originally thought to be, and often s ...
. Without INK4a, Cdk4/6 can inappropriately
phosphorylate In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, wh ...
Rb, leading to increased E2F-dependent transcription. Without ARF, Mdm2 can inappropriately inhibit p53, leading to increased cell survival. The INK4a/ARF locus is found to be deleted or silenced in many kinds of tumors. For example, of the 100 primary breast carcinomas, approximately 41% have p14ARF defects. In a separate study, 32% of colorectal
adenomas An adenoma is a benign tumor of epithelial tissue with glandular origin, glandular characteristics, or both. Adenomas can grow from many glandular organs, including the adrenal glands, pituitary gland, thyroid, prostate, and others. Some adenom ...
(non-cancerous tumors) were found to have p14ARF inactivation due to hyper
methylation In the chemical sciences, methylation denotes the addition of a methyl group on a substrate, or the substitution of an atom (or group) by a methyl group. Methylation is a form of alkylation, with a methyl group replacing a hydrogen atom. These t ...
of the promoter. Mouse models lacking p19Arf, p53, and Mdm2 are more prone to tumor development than mice without Mdm2 and p53, alone. This suggests that p19Arf has Mdm2- and p53-independent effects, as well. Investigating this idea lead to the recent discovery of smARF. Homozygous deletions and other mutations of CDK2NA (ARF) have been found to be associated with
glioblastoma Glioblastoma, previously known as glioblastoma multiforme (GBM), is one of the most aggressive types of cancer that begin within the brain. Initially, signs and symptoms of glioblastoma are nonspecific. They may include headaches, personality ch ...
.


smARF

Until recently, the two known effects of ARF were growth inhibition by NPM interactions and apoptosis induction by Mdm2 interactions. The function of ARF involving
p53 p53, also known as Tumor protein P53, cellular tumor antigen p53 (UniProt name), or transformation-related protein 53 (TRP53) is a regulatory protein that is often mutated in human cancers. The p53 proteins (originally thought to be, and often s ...
-independent death, has now been attributed to the small
mitochondrial A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is use ...
isoform A protein isoform, or "protein variant", is a member of a set of highly similar proteins that originate from a single gene or gene family and are the result of genetic differences. While many perform the same or similar biological roles, some isof ...
of ARF, smARF. While full-length ARF inhibits cell growth by
cell cycle The cell cycle, or cell-division cycle, is the series of events that take place in a cell that cause it to divide into two daughter cells. These events include the duplication of its DNA (DNA replication) and some of its organelles, and subs ...
arrest or type I
apoptotic Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes (morphology) and death. These changes includ ...
death, smARF kills cells by type II autophagic death. Like ARF, the expression of smARF increases when there are aberrant proliferation signals. When smARF is overexpressed, it localizes to the
mitochondrial matrix In the mitochondrion, the matrix is the space within the inner membrane. The word "matrix" stems from the fact that this space is viscous, compared to the relatively aqueous cytoplasm. The mitochondrial matrix contains the mitochondrial DNA, ribo ...
, damaging the mitochondria membrane potential and structure, and leading to autophagic cell death. The translation of the truncated ARF, smARF, is initiated at an internal
methionine Methionine (symbol Met or M) () is an essential amino acid in humans. As the precursor of other amino acids such as cysteine and taurine, versatile compounds such as SAM-e, and the important antioxidant glutathione, methionine plays a critical ro ...
(M45) of the ARF transcript in human and mouse cells. SmARF is also detected in rats, even though an internal methionine is not present in the rat transcript. This suggests that there is an alternate mechanism to form smARF, underscoring the importance of this
isoform A protein isoform, or "protein variant", is a member of a set of highly similar proteins that originate from a single gene or gene family and are the result of genetic differences. While many perform the same or similar biological roles, some isof ...
. The role of smARF is distinct from that of ARF, as it lacks the
nuclear localization signal A nuclear localization signal ''or'' sequence (NLS) is an amino acid sequence that 'tags' a protein for import into the cell nucleus by nuclear transport. Typically, this signal consists of one or more short sequences of positively charged lysines o ...
(NLS) and cannot bind to Mdm2 or NPM. In some cell types, however, full-length ARF can also localize to the
mitochondria A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...
and induce type II cell death, suggesting that in addition to
autophagy Autophagy (or autophagocytosis; from the Ancient Greek , , meaning "self-devouring" and , , meaning "hollow") is the natural, conserved degradation of the cell that removes unnecessary or dysfunctional components through a lysosome-dependent re ...
being a starvation or other environmental response, it may also be involved in responding to
oncogene An oncogene is a gene that has the potential to cause cancer. In tumor cells, these genes are often mutated, or expressed at high levels.
activation.


Biochemistry

ARF expression is regulated by
oncogenic Carcinogenesis, also called oncogenesis or tumorigenesis, is the formation of a cancer, whereby normal cells are transformed into cancer cells. The process is characterized by changes at the cellular, genetic, and epigenetic levels and abno ...
signaling. Aberrant
mitogenic A mitogen is a small bioactive protein or peptide that induces a cell to begin cell division, or enhances the rate of division (mitosis). Mitogenesis is the induction (triggering) of mitosis, typically via a mitogen. The mechanism of action of a ...
stimulation, such as by
MYC ''Myc'' is a family of regulator genes and proto-oncogenes that code for transcription factors. The ''Myc'' family consists of three related human genes: ''c-myc'' (MYC), ''l-myc'' ( MYCL), and ''n-myc'' (MYCN). ''c-myc'' (also sometimes refe ...
or
Ras (protein) Ras, from "Rat sarcoma virus", is a family of related proteins that are expressed in all animal cell lineages and organs. All Ras protein family members belong to a class of protein called small GTPase, and are involved in transmitting signals ...
, will increase its expression, as will an amplification of mutated
p53 p53, also known as Tumor protein P53, cellular tumor antigen p53 (UniProt name), or transformation-related protein 53 (TRP53) is a regulatory protein that is often mutated in human cancers. The p53 proteins (originally thought to be, and often s ...
or Mdm2, or p53 loss. ARF can also be induced by enforced E2F expression. Although E2F expression is increased during the
cell cycle The cell cycle, or cell-division cycle, is the series of events that take place in a cell that cause it to divide into two daughter cells. These events include the duplication of its DNA (DNA replication) and some of its organelles, and subs ...
, ARF expression probably is not because the activation of a second, unknown transcription factor might be needed to prevent an ARF response to transient E2F increases. ARF is negatively regulated by Rb-E2F complexes and by amplified p53 activation. Aberrant growth signals also increase smARF expression. ARF is a highly basic (pI>12) and
hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, th ...
protein. Its basic nature is attributed to its arginine content; more than 20% of its amino acids are arginine, and it contains little or no lysine. Due to these characteristics, ARF is likely to be unstructured unless it is bound to other targets. It reportedly complexes with more than 25 proteins, although the significance of each of these interactions is not known. One of these interactions results in sumoylating activity, suggesting that ARF may modify proteins to which it binds. The SUMO protein is a small
ubiquitin Ubiquitin is a small (8.6 kDa) regulatory protein found in most tissues of eukaryotic organisms, i.e., it is found ''ubiquitously''. It was discovered in 1975 by Gideon Goldstein and further characterized throughout the late 1970s and 1980s. Fo ...
-like modifier, which is added to lysly ε-amino groups. This process involves a three-
enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...
cascade similar to the way
ubiquitylation Ubiquitin is a small (8.6 kDa) regulatory protein found in most tissues of eukaryotic organisms, i.e., it is found ''ubiquitously''. It was discovered in 1975 by Gideon Goldstein and further characterized throughout the late 1970s and 1980s. Fou ...
occurs. E1 is an activating enzyme, E2 is a conjugation enzyme, and E3 is a ligase. ARF associates with UBC9, the only SUMO E2 known, suggesting ARF facilitates SUMO conjugation. The importance of this role is unknown, as
sumoylation In molecular biology, SUMO (Small Ubiquitin-like Modifier) proteins are a family of small proteins that are covalently attached to and detached from other proteins in cells to modify their function. This process is called SUMOylation (sometimes w ...
is involved in different functions, such as protein trafficking, ubiquitylation interference, and gene expression changes. The
half-life Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ato ...
of ARF is about 6 hours, while the half-life of smARF is less than 1 hour. Both
isoforms A protein isoform, or "protein variant", is a member of a set of highly similar proteins that originate from a single gene or gene family and are the result of genetic differences. While many perform the same or similar biological roles, some isof ...
are degraded in the
proteasome Proteasomes are protein complexes which degrade unneeded or damaged proteins by proteolysis, a chemical reaction that breaks peptide bonds. Enzymes that help such reactions are called proteases. Proteasomes are part of a major mechanism by w ...
. ARF is targeted for the
proteasome Proteasomes are protein complexes which degrade unneeded or damaged proteins by proteolysis, a chemical reaction that breaks peptide bonds. Enzymes that help such reactions are called proteases. Proteasomes are part of a major mechanism by w ...
by
N-terminus The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the ami ...
ubiquitylation Ubiquitin is a small (8.6 kDa) regulatory protein found in most tissues of eukaryotic organisms, i.e., it is found ''ubiquitously''. It was discovered in 1975 by Gideon Goldstein and further characterized throughout the late 1970s and 1980s. Fou ...
. Proteins are usually ubiquinated at
lysine Lysine (symbol Lys or K) is an α-amino acid that is a precursor to many proteins. It contains an α-amino group (which is in the protonated form under biological conditions), an α-carboxylic acid group (which is in the deprotonated −C ...
residues. Human p14ARF, however, does not contain any lysines, and mouse p19Arf only contains one lysine. If the mouse lysine is replaced with arginine, there is no effect on its degradation, suggesting it is also ubiquinated at the N-terminus. This adds to the uniqueness of the ARF proteins, because most
eukaryotic Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
proteins are
acetylated : In organic chemistry, acetylation is an organic esterification reaction with acetic acid. It introduces an acetyl group into a chemical compound. Such compounds are termed ''acetate esters'' or simply ''acetates''. Deacetylation is the opposit ...
at the
N-terminus The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the ami ...
, preventing ubiquination at this location. Penultimate residues affect the efficiency of acetylation, in that acetylation is promoted by acidic residues and inhibited by basic ones. The N-terminal amino acid sequences of p19Arf (Met-Gly-Arg) and p14ARF (Met-Val-Arg) would be processed by methionine aminopeptidase but would not be acetylated, allowing ubiquination to proceed. The sequence of smARF, however, predicts that the initiating methionine would not be cleaved by methionine aminopeptidase and would probably be acetylated, and so is degraded by the proteasome without ubiquination. Full-length nucleolar ARF appears to be stabilized by NPM. The NPM-ARF complex does not block the N-terminus of ARF but likely protects ARF from being accessed by degradation machinery. The
mitochondrial matrix In the mitochondrion, the matrix is the space within the inner membrane. The word "matrix" stems from the fact that this space is viscous, compared to the relatively aqueous cytoplasm. The mitochondrial matrix contains the mitochondrial DNA, ribo ...
protein p32 stabilizes smARF. This protein binds various cellular and viral proteins, but its exact function is unknown. Knocking down p32 dramatically decreases smARF levels by increasing its turnover. The levels of p19Arf are not affected by p32 knockdown, and so p32 specifically stabilizes smARF, possibly by protecting it from the
proteasome Proteasomes are protein complexes which degrade unneeded or damaged proteins by proteolysis, a chemical reaction that breaks peptide bonds. Enzymes that help such reactions are called proteases. Proteasomes are part of a major mechanism by w ...
or from
mitochondrial A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is use ...
proteases A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalyzes (increases reaction rate or "speeds up") proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the for ...
.


References

Zhang, Y., Y. Xiong, and W.G. Yarbrough. ARF Promotes MDM2 Degradation and Stabilizes p53: ARF-INK4a Locus Deletion Impairs Both the Rb and p53 Tumor Suppression Pathways. Cell 1998, 92(6):725-34.


Further reading

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External links

* {{Cell cycle proteins