Ataxin-1 is a DNA-binding

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

which in humans is encoded by the ''ATXN1''

gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

. Mutations in ataxin-1 cause

spinocerebellar ataxia type 1 Spinocerebellar ataxia type 1 (SCA1) is a rare autosomal dominant disorder, which, like other spinocerebellar ataxias, is characterized by neurological symptoms including dysarthria, hypermetric saccades, and ataxia of gait and stance. This cer ...

, an inherited

neurodegenerative A neurodegenerative disease is caused by the progressive loss of neurons, in the process known as neurodegeneration. Neuronal damage may also ultimately result in their death. Neurodegenerative diseases include amyotrophic lateral sclerosis, mul ...

disease characterized by a progressive loss of cerebellar neurons, particularly Purkinje neurons.

Genetics

''ATXN1'' is conserved across multiple species, including humans, mice, and ''Drosophila.'' In humans, ''ATXN1'' is located on the short arm of

chromosome 6 Chromosome 6 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 6 spans nearly 171 million base pairs (the building material of DNA) and represents between 5.5 and 6% of the total DNA i ...

. The gene contains 9

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 i ...

, two of which are protein-coding. There is a CAG repeat in the coding sequence which is longer in humans than other species (6-38 uninterrupted CAG repeats in healthy humans versus 2 in the mouse gene). This repeat is prone to errors in DNA replication and can vary widely in length between individuals.

Structure

Notable features of the Ataxin-1 protein structure include: * A

polyglutamine tract A polyglutamine tract or polyQ tract is a portion of a protein consisting of a sequence of several glutamine units. A tract typically consists of about 10 to a few hundred such units. A multitude of genes, in various eukaryotic species (including ...

of variable length, encoded by the CAG repeat in ''ATXN1.'' * A region which mediates protein-protein interactions, known as the AXH domain * A

nuclear localization sequence 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 ...

* A

phosphorylation In biochemistry, phosphorylation is described as the "transfer of a phosphate group" from a donor to an acceptor. A common phosphorylating agent (phosphate donor) is ATP and a common family of acceptor are alcohols: : This equation can be writ ...

site which regulates the protein's stability and interactions with its binding partners

Function

The function of Ataxin-1 is not completely understood. It appears to be involved in regulating

gene expression Gene expression is the process (including its Regulation of gene expression, regulation) by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, proteins or non-coding RNA, ...

based on its location in the nucleus of the cell, its association with promoter regions of several genes, and its interactions with transcriptional regulators and parts of the

RNA splicing RNA splicing is a process in molecular biology where a newly-made precursor messenger RNA (pre-mRNA) transcription (biology), transcript is transformed into a mature messenger RNA (Messenger RNA, mRNA). It works by removing all the introns (non-cod ...

machinery.

Interactions

Ataxin 1 has been shown to

interact Advocates for Informed Choice, dba interACT or interACT Advocates for Intersex Youth, is a 501(c)(3) nonprofit organization advocating for the legal and human rights of children with intersex traits. The organization was founded in 2006 and fo ...

with: *

C2orf27 Uncharacterized protein C2orf27 is a protein that in humans is encoded by the ''C2orf27A gene''. Although its function is not clearly understood, through the use of bioinformatic analysis more information is being brought to light. Gene The mR ...

, *

Coilin Coilin is a protein that in humans is encoded by the ''COIL'' gene. Coilin got its name from the coiled shape of the Cajal bodies in which it is found. It was first identified using human autoimmune serum. Function Coilin protein is one of th ...

, *

Glyceraldehyde 3-phosphate dehydrogenase Glyceraldehyde 3-phosphate dehydrogenase (abbreviated GAPDH) () is an enzyme of about 37kDa that catalyzes the sixth step of glycolysis and thus serves to break down glucose for energy and carbon molecules. In addition to this long establis ...

, *

CIC CIC may refer to: Organizations Canada * Cadet Instructors Cadre, a part of the Canadian Armed Forces * Canadian Infantry Corps, renamed in 1947 to Royal Canadian Infantry Corps * Canadian International Council * Canadian Islamic Congress * Chemi ...

UBE2E1 Ubiquitin-conjugating enzyme E2 E1 is a protein that in humans is encoded by the ''UBE2E1'' gene. Function The modification of proteins with ubiquitin is an important cellular mechanism for targeting abnormal or short-lived proteins for degra ...

, and *

USP7 Ubiquitin-specific-processing protease 7 (USP7), also known as ubiquitin carboxyl-terminal hydrolase 7 or herpesvirus-associated ubiquitin-specific protease (HAUSP), is an enzyme that in humans is encoded by the ''USP7'' gene. Function Regul ...

Role in disease

''ATXN1'' is the gene mutated in

(SCA1), a dominantly-inherited, fatal genetic disease in which neurons in the

cerebellum The cerebellum (: cerebella or cerebellums; Latin for 'little brain') is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as it or eve ...

and

brain stem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is co ...

degenerate over the course of years or decades. SCA1 is a

trinucleotide repeat disorder In genetics, trinucleotide repeat disorders, a subset of microsatellite expansion diseases (also known as repeat expansion disorders), are a set of over 30 genetic disorders caused by trinucleotide repeat expansion, a kind of mutation in which ...

caused by expansion of the CAG repeat in ''ATXN1''; this leads to an expanded polyglutamine tract in the protein. This elongation is variable in length, with as few as 6 and as many as 81 repeats reported in humans. Repeats of 39 or more uninterrupted CAG triplets cause disease, and longer repeat tracts are correlated with earlier age of onset and faster progression. How polyglutamine expansion in Ataxin-1 causes neuronal dysfunction and degeneration is still unclear. Disease likely occurs through the combination of several processes.

Aggregation

Mutant Ataxin-1 protein spontaneously misfolds and forms aggregates in cells, much like other disease-associated proteins such as

tau Tau (; uppercase Τ, lowercase τ or \boldsymbol\tau; ) is the nineteenth letter of the Greek alphabet, representing the voiceless alveolar plosive, voiceless dental or alveolar plosive . In the system of Greek numerals, it has a value of 300 ...

, Aβ, and

huntingtin Huntingtin (Htt) is the protein coded for in humans by the ''HTT'' gene, also known as the ''IT15'' ("interesting transcript 15") gene. Mutation, Mutated ''HTT'' is the cause of Huntington's disease (HD), and has been investigated for this role an ...

. This led to the hypothesis that the aggregates are toxic to neurons, but it has been shown in mice that aggregation is not required for pathogenesis. Other neuronal proteins can modulate the formation of Ataxin-1 aggregates and this in turn may affect aggregate-induced toxicity.

Altered protein-protein interactions

Soluble Ataxin-1 interacts with many other proteins. Polyglutamine expansion in Ataxin-1 can affect these interactions, sometimes causing loss of function (where the protein fails to perform one of its normal functions) and sometimes causing toxic gain of function (where the protein binds too strongly or to an inappropriate target). This, in turn, could alter the expression of the genes ataxin-1 regulates, leading to disease.

HMGB1 High mobility group box 1 protein, also known as high-mobility group protein 1 (HMG-1) and amphoterin, is a protein that in humans is encoded by the ''HMGB1'' gene. HMG-1 belongs to the high mobility group and contains a HMG-box domain. Funct ...
interaction

Mutant ataxin1 causes the

neurodegenerative disease A neurodegenerative disease is caused by the progressive loss of neurons, in the process known as neurodegeneration. Neuronal damage may also ultimately result in their death. Neurodegenerative diseases include amyotrophic lateral sclerosis, mul ...

(SCA1). In a mouse model of SCA1, mutant ataxin1 mediates the reduction or inhibition of the high mobility group box1 protein (

HMGB1 High mobility group box 1 protein, also known as high-mobility group protein 1 (HMG-1) and amphoterin, is a protein that in humans is encoded by the ''HMGB1'' gene. HMG-1 belongs to the high mobility group and contains a HMG-box domain. Funct ...

) in

neuron A neuron (American English), neurone (British English), or nerve cell, is an membrane potential#Cell excitability, excitable cell (biology), cell that fires electric signals called action potentials across a neural network (biology), neural net ...

mitochondria 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 us ...

. HMGB1 is a crucial nuclear protein that regulates

DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...

architectural changes essential for

DNA damage repair DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. A weakened capacity for DNA repair is a risk factor for the development of cancer. DNA is constantly modified ...

and

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, often th ...

. The impairment of HMGB1 function leads to increased

mitochondrial DNA Mitochondrial DNA (mtDNA and mDNA) is the DNA located in the mitochondrion, mitochondria organelles in a eukaryotic cell that converts chemical energy from food into adenosine triphosphate (ATP). Mitochondrial DNA is a small portion of the D ...

damage. In the SCA1 mouse model, over-expression of the HMGB1 protein by means of an introduced virus vector bearing the HMGB1 gene facilitates repair of the mitochondrial DNA damage, ameliorates the

neuropathology Neuropathology is the study of disease of nervous system tissue, usually in the form of either small surgical biopsies or whole-body autopsies. Neuropathologists usually work in a department of anatomic pathology, but work closely with the clini ...

and the motor deficits, and extends the lifespan of these mutant ataxin1 mice.

References

External links

GeneReviews/NIH/NCBI/UW entry on Spinocerebellar Ataxia Type 1
* * * {{NLM content Proteins