G-proteins are a family of regulatory proteins involved in both intra- and intercellular transduction processes. They are heterotrimeric, consisting of three sub units (alpha, beta, gamma), and share structural homology. G-protein-coupled receptors comprise an extracellular ligand binding site and an effector site that extends into the cytosol.
In the resting state, GDP is bound to the alpha subunit, which has GTPase activity at the guanine nucleotide-binding site and separate binding sites for the receptor and effector proteins. The beta and gamma units are shared among G-proteins.
Membrane receptor protein super-families
G-protein-coupled seven-transmembrane (GPCR) proteins S
LH, FSH, thyroid stimulating hormone (TSH) o
Parathyroid hormone (PTH), parathyroid hormone releasing peptide (PTHrP) m
Adrenocorticotrophic hormone (ACTH), melanocyte stimulating hormone (MSH)
Growth hormone releasing hormone (GHRH), corticotrophin releasing hormone "
(CRH) Alpha-adrenergic Somatostatin
Thyrotrophin releasing hormone (TRH), gonadotrophin releasing hormone (GnRH)
Receptor tyrosine kinase (phosphorylate tyrosines of target molecules)
Insulin, insulin-like growth factor-1 (IGF-1) Epidermal growth factor, nerve growth factor
Cytokine receptor-linked kinase
Growth hormone (GH), prolactin (PRL)
Serine kinase (phosphorylate serines and threonines of signalling molecules)
Activin, tumour growth factor (TGF)-beta, MIS
Guanylate cyclase (form cyclic GMP in the cytosol)
Atrial natriuretic peptides
Ligand-gated ion channel
The alpha subunit confers functional and binding specificity to the various types of G-proteins. It is unique for each G-protein. On hormone binding to the receptor, GDP rapidly exchanges for guanosine 5'-triphosphate (GTP) on the alpha subunit, activating the G-protein. The G-protein dissociates from the receptor. The alpha subunit and beta-gamma complex dissociate to interact with other effector proteins. The alpha subunit modulates the activity of a second messenger system. Guanidine triphosphate is then rapidly hydrolysed to GDP by the GTPase activity of the alpha subunit, with the release of inorganic phosphate. The G-protein heterotrimeric complex is thereupon re-formed. The two control points in the G-protein cycle are GDP release from the alpha subunit and hydrolysis of GTP catalysed by the alpha subunit.
G-proteins may be either coupled to adenyl cyclase via either Gs (increased cyclic AMP) or Gi (reduced cyclic AMP); or coupled to Gq or G11 and convert phosphatidylinositol 4,5-biphosphate into two second messengers, inositol 1,4,5-triphosphate (which increases intracellular Ca2+ concentration by causing release from endoplasmic reticulum) and diacylglycerol (which activates protein kinase C).
Properties of G-protein-coupled receptors e They are monomeric proteins of the rhodopsin family, which sense external environ-
h mental signals such as light, taste and odour, and signals transmitted by hormones.
i They possess seven membrane spanning domains with four intracellular loops respon-
° sible for direct interaction with the G-protein. There is an N-terminal extracellular ig domain and a C-terminal intracellular domain.
Increases in concentration following binding of hormone to receptor; An increase in messenger concentration precedes biological effects of hormone; When the hormone is removed the concentration of the second messenger and the biological response proportionately decline.
Second messenger (intracellular mediator) pathways
Diacylglycerol-IP3 (inositol-1,4,5-triphosphate) Arachidonic acid Cyclic GMP Tyrosine kinases
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