Gamma-aminobutyric acid receptor subunit alpha-3 is a protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 that in humans is encoded by the GABRA3 gene

Gene

A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

.

Function

GABA

Gabâ

Gabâ or gabaa, for the people in many parts of the Philippines), is the concept of a non-human and non-divine, imminent retribution. A sort of negative karma, it is generally seen as an evil effect on a person because of their wrongdoings or transgressions...

 is the major inhibitory neurotransmitter in the mammalian brain where it acts at GABA_A receptors, which are ligand-gated chloride channel

Chloride channel

Chloride channels are a superfamily of poorly understood ion channels consisting of approximately 13 members.Chloride channels display a variety of important physiological and cellular roles that include regulation of pH, volume homeostasis, organic solute transport, cell migration, cell...

s. Chloride conductance of these channels can be modulated by agents such as benzodiazepine

Benzodiazepine

A benzodiazepine is a psychoactive drug whose core chemical structure is the fusion of a benzene ring and a diazepine ring...

s that bind to the GABA_A receptor. At least 16 distinct subunits of GABA-A receptors have been identified. GABA receptors are composed of 5 subunits with an extracellular ligand binding domains and ion channel domains that are integral to the membrane.Ligand binding to these receptors activates the channel.

Subunit selective ligands

Recent research has produced several ligands which are moderately selective for GABA_A receptors containing the α₃ subunit. Subtype-selective agonists for α₃ produce anxiolytic

Anxiolytic

An anxiolytic is a drug used for the treatment of anxiety, and its related psychological and physical symptoms...

 and mild sedative

Sedative

A sedative or tranquilizer is a substance that induces sedation by reducing irritability or excitement....

 effects, but without causing amnesia

Amnesia

Amnesia is a condition in which one's memory is lost. The causes of amnesia have traditionally been divided into categories. Memory appears to be stored in several parts of the limbic system of the brain, and any condition that interferes with the function of this system can cause amnesia...

 or ataxia

Ataxia

Ataxia is a neurological sign and symptom that consists of gross lack of coordination of muscle movements. Ataxia is a non-specific clinical manifestation implying dysfunction of the parts of the nervous system that coordinate movement, such as the cerebellum...

, which could make them superior to currently marketed drugs.

Agonists

• Adipiplon
  Adipiplon
  Adipiplon is an anxiolytic drug developed by Neurogen Corporation. It has similar effects to benzodiazepine drugs, but is structurally distinct and so is classed as a nonbenzodiazepine anxiolytic....
  
  
• PWZ-029
  PWZ-029
  PWZ-029 is a benzodiazepine derivative drug with nootropic effects. It acts as a subtype-selective, mixed agonist-inverse agonist at the benzodiazepine binding site on the GABAA receptor, acting as a partial inverse agonist at the α5 subtype and a weak partial agonist at the α3 subtype...
  
   (partial agonist at α₃, partial inverse agonist at α₅)

RNA editing

The GABRA3 transcript undergoes pre-mRNA editing by the ADAR

ADAR

Double-stranded RNA-specific adenosine deaminase is an enzyme that in humans is encoded by the ADAR gene.-Further reading:...

 family of enzymes. A-to-I editing changes an isoleucine

Isoleucine

Isoleucine is an α-amino acid with the chemical formula HO2CCHCHCH2CH3. It is an essential amino acid, which means that humans cannot synthesize it, so it must be ingested. Its codons are AUU, AUC and AUA....

 codon to code for a methionine

Methionine

Methionine is an α-amino acid with the chemical formula HO2CCHCH2CH2SCH3. This essential amino acid is classified as nonpolar. This amino-acid is coded by the codon AUG, also known as the initiation codon, since it indicates mRNA's coding region where translation into protein...

 residue. This editing is thought to be important for brain development as the level of editing is low at birth and becomes almost 100% in an adult brain.

The editing occurs in an RNA stem-loop

Stem-loop

Stem-loop intramolecular base pairing is a pattern that can occur in single-stranded DNA or, more commonly, in RNA. The structure is also known as a hairpin or hairpin loop. It occurs when two regions of the same strand, usually complementary in nucleotide sequence when read in opposite directions,...

 found in exon

Exon

An exon is a nucleic acid sequence that is represented in the mature form of an RNA molecule either after portions of a precursor RNA have been removed by cis-splicing or when two or more precursor RNA molecules have been ligated by trans-splicing. The mature RNA molecule can be a messenger RNA...

 9. The structured loci was identified using a specialised bioinformatics

Bioinformatics

Bioinformatics is the application of computer science and information technology to the field of biology and medicine. Bioinformatics deals with algorithms, databases and information systems, web technologies, artificial intelligence and soft computing, information and computation theory, software...

 screen of the human genome. The proposed function of the edit is to alter chloride

Chloride

The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...

 permeability of the GABA receptor

GABA receptor

The GABA receptors are a class of receptors that respond to the neurotransmitter gamma-aminobutyric acid , the chief inhibitory neurotransmitter in the vertebrate central nervous system...

.

At the time of discovery, Kv1.1 mRNA was the only previously known mammalian coding site containing both the edit sequence and the editing complementary sequence.

Type

A to I RNA editing is catalyzed by a family of adenosine deaminase

Adenosine deaminase

Adenosine deaminase is an enzyme involved in purine metabolism. It is needed for the breakdown of adenosine from food and for the turnover of nucleic acids in tissues.-Reactions:...

s acting on RNA (ADARs) that specifically recognize adenosines within double-stranded regions of pre-mRNAs and deaminate them to inosine

Inosine

Inosine is a nucleoside that is formed when hypoxanthine is attached to a ribose ring via a β-N9-glycosidic bond....

. Inosines are recognised as guanosine

Guanosine

Guanosine is a purine nucleoside comprising guanine attached to a ribose ring via a β-N9-glycosidic bond. Guanosine can be phosphorylated to become guanosine monophosphate , cyclic guanosine monophosphate , guanosine diphosphate , and guanosine triphosphate...

 by the cells translational machinery. There are three members of the ADAR family ADARs 1-3 with ADAR1

ADAR

Double-stranded RNA-specific adenosine deaminase is an enzyme that in humans is encoded by the ADAR gene.-Further reading:...

 and ADAR2

ADARB1

Double-stranded RNA-specific editase 1 is an enzyme that in humans is encoded by the ADARB1 gene.ADAR2 requires the small molecule inositol hexakisphosphate for proper function.-Further reading:...

 being the only enzymatically active members. ADAR3

ADARB2

Double-stranded RNA-specific editase B2 is an enzyme that in humans is encoded by the ADARB2 gene.-Further reading:...

 is thought to have a regulatory role in the brain. ADAR1 and ADAR 2 are widely expressed in tissues while ADAR3 is restricted to the brain. The double stranded regions of RNA are formed by base-pairing between residues in the close to region of the editing site with residues usually in a neighboring intron but can be an exonic sequence. The region that base pairs with the editing region is known as an Editing Complementary Sequence (ECS).

Location

The editing site was previously believed to be a sing nucleotide polymorphism. The editing site is found iat amino acid 5 of transmembrane domain 3 of exon 9. The predicted double stranded RNA structure is interrupted by three bulges and a mismatch at the editing site.The double stranded region is 22 base pairs in length. Like for editing of the KCNA1 gene product the editing region and the editing complemtary sequence are both found in exonic regions. In the pre=mRNA of GABRA3 both are found within exon 9. The other subunits of the receptor are thought not to be edited as their predicted secondary structure are less likely to be edited. Also alpha subunits 1 and 6 have a uridine instead of an adenosine at the site orresponding to the editing site in alpha subunit 3. Point mutation experiments determined that a Cytidine 15 nucleotides from the editing site is the base opposite the edited base. Using a GABRA3 mini gene which encodes for exon 9 cotransfected to HEK293 cells wuth either ADAR1 or 2 or none, it was determined that both active ADARs can efficiently edited the site in exon 9.

Regulation

The mRNA expression of the alpha 3 subunit is developmentally regulated.It is the dominant subunit in the forebrain tissue at birth gradually decreasing in promenance as alpha subunit 1 takes over. Also experiments with mice have demonstrated that editing of pre-mRNA alpha 3 subunit increases fom 50% at birth to nearly 100% in adult. Editing levels are lower in the hippocampus

Conservation

At the location corresponding to the I/M site of GABRA3 in frog and pufferfish there is a genomically encoded methionine.In all other species there is an isoleucine at the position.

Structure

Editing results in a codon change from (AUA)I to (AUG)M at the editing site.This results in translation of a methionine instead of an isoleucine at the I/M site.The amino acid change occurs in the transmembrane domain 3. The 4 transmembrane domains of each of the 5 subunits that make up the receptor interact to form the receptor channel.It is likely change of amino acid make disturb the structure effecting gating and inactivation of the channel. This is because methionine has a larger side chain.

Function

While the effect of editing on protein function is unknown, the developmental increase in editing does correspond to changes in function of the GAGA_A receptor. GABA binding leads to chloride channel activation resulting in rapid increase in concentration of the ion.Initially the receptor is an excitatory receptor, mediating depolarisation (efflux of Cl^- ions) in immature neurons before changing to an inhibitory receptor, mediating hyperpolarisation(influx of Cl^- ions) later on. GABA_A converts to an inhibitory receptor from an excitatory receptor by the upregulation of KCC2 cotransporter. This decreases the concentration of Cl^- ion within cells. Therefore the GAGA_A subunits are involved in determining the nature of the receptor in response to GABA ligand. These changes suggest that editing of the subunit is important in the developing brain by regulating the Cl^- permeability of the channel during development. The unedited receptor is activated faster and deactivates slower than the edited receptor.