Gamma-aminobutyric acid receptor subunit beta-3 is a protein that in humans is encoded by the GABRB3 gene. It is located within the 15q12 region in the human genome and spans 250kb. This gene includes 10 exons within its coding region. Due to alternative splicing, the gene codes for many protein isoforms, all being subunits in the GABAA receptor, a ligand-gated ion channel. The beta-3 subunit is expressed at different levels within the cerebral cortex, hippocampus, cerebellum, thalamus, olivary body and piriform cortex of the brain at different points of development and maturity. GABRB3 deficiencies are implicated in many human neurodevelopmental disorders and syndromes such as Angelman syndrome, Prader-Willi syndrome, nonsyndromic orofacial clefts, epilepsy and autism. The effects of methaqualone and etomidate are mediated through GABBR3 positive allosteric modulation.
Gene
The GABRB3 gene is located on the long arm of chromosome 15, within the q12 region in the human genome. It is located in a gene cluster, with two other genes, GABRG3 and GABRA5. GABRB3 was the first gene to be mapped to this particular region. It spans approximately 250kb and includes 10 exons within its coding region, as well as two additional alternative first exons that encode for signaling peptides. Alternatively spliced transcript variants encoding isoforms with distinct signal peptides have been described. This gene is located within an imprinting region that spans the 15q11-13 region. Its sequence is considerably longer than the two other genes found within its gene cluster due to a large 150kb intron it carries. A pattern is observed in GABRB3 gene replication, in humans the maternal allele is replicated later than the paternal allele. The reasoning and implications of this pattern are unknown.
When comparing the human beta-3 subunit's genetic sequence with other vertebrate beta-3 subunit sequences, there is a high level of genetic conservation. In mice the Gabrb3 gene is located on chromosome 7 of its genome in a similar gene cluster style with some of the other subunits of the GABAA receptor.
Function
GABRB3 encodes a member of the ligand-gated ion channel family. The encoded protein is one of at least 13 distinct subunits of a multisubunit chloride channel that serves as the receptor for gamma-aminobutyric acid, the major inhibitory neurotransmitter of the nervous system. The two other genes in the gene cluster both encode for related subunits of the family. During development, when the GABRB3 subunit functions optimally, its role in the GABAA receptor allows for proliferation, migration, and differentiation of precursor cells that lead to the proper development of the brain. GABAA receptor function is inhibited by zinc ions. The ions bind allosterically to the receptor, a mechanism that is critically dependent on the receptor subunit composition.
De novo heterozygous missense mutations within a highly conserved region of the GABRB3 gene can decrease the peak current amplitudes of neurons or alter the kinetic properties of the channel. This results in the loss of the inhibitory properties of the receptor.
The beta-3 subunit has very similar function to the human version of the subunit.
Structure
The crystal structure of a human β3 homopentamer was published in 2014. The study of the crystal structure of the human β3 homopentamer revealed unique qualities that are only observed in eukaryotic cysteine-loop receptors. The characterization of the GABAA receptor and subunits helps with the mechanistic determination of mutations within the subunits and what direct effect the mutations may have on the protein and its interactions.