guanine nucleotide-binding protein G(t) subunit alpha-3 (gustducin alpha-3 chain, GNAT3)
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Function
- G-alpha
- role in bitter & sweet taste transduction as well as in umami (monoNa+ glutamate, monoK+ glutamate, & inosine monophosphate) taste transduction
- transduction involves coupling of cell-surface receptors with a cGMP-phosphodiesterase
- activation of phosphodiesterase lowers intracellular levels of cAMP & cGMP which may open a cyclic nucleotide-suppressible cation channel leading to influx of Ca+2, ultimately leading to release of neurotransmitter
- gustducin heterotrimer transduces response to bitter & sweet compounds via regulation of phosphodiesterase for alpha subunit, as well as via activation of phospholipase C for beta & gamma subunits, with ultimate increase inositol trisphosphate & increase of intracellular Ca+2
- GNAT3 can functionally couple to taste receptors to transmit intracellular signal:
- also functions as lumenal sugar sensors in the gut to control:
- expression of the Na+-glucose transporter SGLT1 in response to dietaty sugar
- secretion of glucagon-like peptide-1, GLP-1 & glucose-dependent insulinotropic polypeptide, GIP
- may modulate the gut capacity to absorb sugars, with implications in malabsorption syndromes & diet-related disorders including diabetes & obesity
Structure
- belongs to the G-alpha family, G(i/o/t/z) subfamily
- gustducin heterotrimer may also be composed of GNAT3, GNB3 & GNG13
- N-myristoylation may anchor alpha-subunit to the inner surface of plasma membrane (putative)
Compartment
- cytoplasm
- dual ditribution pattern:
- plasmalemmal pattern with apical region localization
- cytosolic pattern with localization throughout cytoplasm
Expression
- expressed in taste buds (sensory organs of clustered epithelial cells) of the circumvallate & foliate papillae of the tongue at protein level
- expressed in enteroendocrine L cells of the gut
- detected also in spermatozoa