transmembrane domain of alpha subunits of glycine receptor (GlyR)
This family contains transmembrane (TM) domain of the alpha subunit of glycine receptor (GlyR or GLR) of the amino acid neurotransmitter glycine. The transmembrane region consists of four transmembrane-spanning alpha-helical segments (M1-M4) that are linked by loops. The intracellular loop that links M1 and M2 determines the ion selectivity of the channel. GlyR has four known isoforms of the alpha-subunit (alpha1-4, encoded by GLRA1, GLRA2, GLRA3, GLRA4) that are essential to bind ligands and, along with the GlyR beta subunit, have been described to have a regionally and temporally controlled expression during development and maturation of the central nervous system (CNS). These alpha subunits are highly homologous but differ in their kinetic properties, temporal and regional expression and physiological functions. They can form functional chloride-permeable GlyR ion channels by forming homopentamers with 5 alpha subunits or heteropentamers with a combination of alpha and beta subunits, either a 2alpha-3beta or 3alpha-2beta stoichiometry. In human, mutations in glycine receptor alpha subunits cause disruption of GlyR surface expression or reduced ability of expressed GlyRs to conduct chloride ions. Mutations in GlyR alpha1 subunit leads to hyperekplexia, a rare neurological disorder characterized by neonatal hypertonia and exaggerated startle responses to unexpected stimuli, while mutations in GlyR alpha2 are known to cause cortical neuronal migration/autism spectrum disorder and in GlyR alpha3 to cause inflammatory pain sensitization/rhythmic breathing. GlyR alpha1 and alpha2 subunits have an important role in regulation of the excitatory-inhibitory balance, control of motor actions, modulation of sedative ethanol effects and probably regulation of ethanol preference and consumption.
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