"G protein" usually refers to the membrane-associated heterotrimeric G proteins, sometimes referred to as the "large" G proteins. These proteins are activated by G protein-coupled receptor

G protein-coupled receptor

G protein-coupled receptors , also known as seven-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptor, and G protein-linked receptors , comprise a large protein family of transmembrane receptors that sense molecules outside the cell and activate inside signal...

s and are made up of alpha (α), beta (β) and gamma (γ) subunit

Protein subunit

In structural biology, a protein subunit or subunit protein is a single protein molecule that assembles with other protein molecules to form a protein complex: a multimeric or oligomeric protein. Many naturally occurring proteins and enzymes are multimeric...

s, the latter two referred to as the beta-gamma complex

Beta-gamma complex

]]The beta-gamma complex is composed of the heterotrimeric G proteins Gβ and Gγ that are closely bound to one another....

.

Alpha subunits

G_α subunits consist of two domains, the GTPase domain, and the alpha-helical

Alpha helix

A common motif in the secondary structure of proteins, the alpha helix is a right-handed coiled or spiral conformation, in which every backbone N-H group donates a hydrogen bond to the backbone C=O group of the amino acid four residues earlier...

 domain. There exist at least 20 different G_α subunits, which are separated into four main families. This nomenclature is based on their sequence homologies:

G-protein-family	α-subunit	Gene	Signal transduction	Use/Receptors (examples)	Effects (examples)
Gi-family
Gi/o	αi Gi alpha subunit Gi alpha subunit is a heterotrimeric G protein subunit that inhibits the production of cAMP from ATP.- Receptors :The following G protein-coupled receptors couple to the Gi subunit:* Acetylcholine M2 & M4 receptors... , αo	GNAO1 GNAO1 Guanine nucleotide-binding protein G subunit alpha is a protein that in humans is encoded by the GNAO1 gene.-Interactions:GNAO1 has been shown to interact with RGS20, RGS19, RIC8A and RGS5.-Further reading:... , GNAI1 GNAI1 Guanine nucleotide-binding protein G, alpha-1 subunit is a protein that in humans is encoded by the GNAI1 gene.-Interactions:GNAI1 has been shown to interact with GPR143, RGS14, RIC8A, S1PR1, RGS12 and RGS19.-Further reading:... , GNAI2 GNAI2 Guanine nucleotide-binding protein G, alpha-2 subunit is a protein that in humans is encoded by the GNAI2 gene.-Interactions:GNAI2 has been shown to interact with GPSM2, RIC8A, Interleukin 8 receptor, alpha, EYA2, RGS5 and MDFI.-Further reading:... ,	Inhibition of adenylate cyclase Adenylate cyclase Adenylate cyclase is part of the G protein signalling cascade, which transmits chemical signals from outside the cell across the membrane to the inside of the cell .... , opens K+-channels (via β/γ subunits), closes Ca2+-channels	Muscarinic M2 and M4, chemokine receptors, α2-Adrenoreceptors Alpha-2 adrenergic receptor The alpha-2 adrenergic receptor is a G protein-coupled receptor associated with the Gi heterotrimeric G-protein. It consists of three highly homologous subtypes, including α2A-, α2B-, and α2C-adrenergic. Some species other than humans express a fourth α2D-adrenergic receptor as well... , Serotonin 5-HT1 receptors, Histamine H3 and H4, Dopamine D2-like receptors	Smooth muscle contraction, depress neuronal activity
Gt	αt (Transducin Transducin Transducin is a heterotrimeric G protein that is naturally expressed in vertebrate retina rods and cones .- Mechanism of action :... )	GNAT1 GNAT1 Guanine nucleotide-binding protein G subunit alpha-1 is a protein that in humans is encoded by the GNAT1 gene.-Further reading:... , GNAT2 GNAT2 Guanine nucleotide-binding protein G subunit alpha-2 is a protein that in humans is encoded by the GNAT2 gene.- Function :Transducin is a 3-subunit guanine nucleotide-binding protein which stimulates the coupling of rhodopsin and cGMP-phoshodiesterase during visual impulses. The transducin alpha...	Activation of phosphodiesterase Phosphodiesterase A phosphodiesterase is any enzyme that breaks a phosphodiester bond. Usually, people speaking of phosphodiesterase are referring to cyclic nucleotide phosphodiesterases, which have great clinical significance and are described below...  6	Rhodopsin Rhodopsin Rhodopsin, also known as visual purple, is a biological pigment of the retina that is responsible for both the formation of the photoreceptor cells and the first events in the perception of light. Rhodopsins belong to the G-protein coupled receptor family and are extremely sensitive to light,...	Vision
Ggust	αgust (Gustducin Gustducin Gustducin is a G protein associated with basic taste and the gustatory system. Due to its relatively recent discovery and isolation, not all is known about its nature and its associated pathways. It is known that it plays a large role in the transduction of bitter, sweet and umami stimuli and... )	GNAT3 Gustducin Gustducin is a G protein associated with basic taste and the gustatory system. Due to its relatively recent discovery and isolation, not all is known about its nature and its associated pathways. It is known that it plays a large role in the transduction of bitter, sweet and umami stimuli and...	Activation of phosphodiesterase 6	Taste receptors	Taste
Gz	αz	GNAZ GNAZ Guanine nucleotide-binding protein G subunit alpha is a protein that in humans is encoded by the GNAZ gene.-Interactions:GNAZ has been shown to interact with EYA2, RGS20 and RGS19.-Further reading:...	Inhibition of adenylate cyclase	?	Maintaining the ionic balance of perilymphatic and endolymphatic cochlear fluids.
Gs-family
Gs	αs Gs alpha subunit The Gs alpha subunit is a heterotrimeric G protein subunit that activates the cAMP-dependent pathway by activating adenylate cyclase.-Receptors:The G protein-coupled receptors that couple to this kind of G-protein include:...		Activation of adenylate cyclase	Beta-adrenoreceptors; Serotonin 5-HT4, 5-HT6 and 5-HT7; Dopamine D1-like receptors, Histamine H2	Increase heart rate, Smooth muscle relaxation, stimulate neuronal activity
Golf	αolf	GNAL GNAL Guanine nucleotide-binding protein G subunit alpha is a protein that in humans is encoded by the GNAL gene.-Further reading:...	Activation of adenylate cyclase	olfactory receptor Olfactory receptor Olfactory receptors expressed in the cell membranes of olfactory receptor neurons are responsible for the detection of odor molecules. Activated olfactory receptors are the initial player in a signal transduction cascade which ultimately produces a nerve impulse which is transmitted to the brain... s	Smell
Gq-family
Gq	αq Gq alpha subunit Gq protein or Gq/11 is a heterotrimeric G protein subunit that activates phospholipase C . PLC in turn hydrolyzes Phosphatidylinositol 4,5-bisphosphate to diacyl glycerol and inositol triphosphate signal transduction pathway... , α11, α14, α15, α16	GNAQ GNAQ Guanine nucleotide-binding protein G subunit alpha is a protein that in humans is encoded by the GNAQ gene.-Interactions:GNAQ has been shown to interact with Sodium-hydrogen antiporter 3 regulator 1, RIC8A, RGS16, Beta adrenergic receptor kinase, RGS4 and Bruton's tyrosine kinase.-Further reading:... , GNA11 GNA11 Guanine nucleotide-binding protein subunit alpha-11 is a protein that in humans is encoded by the GNA11 gene.-Further reading:... , ,	Activation of phospholipase C Phospholipase C Phosphoinositide phospholipase C is a family of eukaryotic intracellular enzymes that play an important role in signal transduction processes. In general, this enzyme is denoted as Phospholipase C, although three other families of phospholipase C enzymes have been identified in bacteria and in...	α1-Adrenoreceptors Alpha-1 adrenergic receptor The alpha-1 adrenergic receptor is a G protein-coupled receptor associated with the Gq heterotrimeric G-protein. It consists of three highly homologous subtypes, including α1A-, α1B-, and α1D-adrenergic... , Histamine H1, Serotonin 5-HT2 receptors, Muscarinic M1 M3, and M5	Smooth muscle contraction, Ca2+ flux
G12/13-family
G12/13	α12, α13	,	Activation of the Rho family of GTPases Rho family of GTPases The Rho family of GTPases is a family of small signaling G protein , and is a subfamily of the Ras superfamily. The members of the Rho GTPase family have been shown to regulate many aspects of intracellular actin dynamics, and are found in all eukaryotic organisms including yeasts and some plants...		Cytoskelatal functions, Smooth muscle contraction

Beta-gamma complex

The β and γ subunits are closely bound to one another and are referred to as the beta-gamma complex

Beta-gamma complex

]]The beta-gamma complex is composed of the heterotrimeric G proteins Gβ and Gγ that are closely bound to one another....

. Upon activation of the GPCR, the G_βγ complex is released from the G_α subunit after its GDP-GTP exchange.

Function

The free G_βγ complex can act as a signaling molecule itself, by activating other second messengers or by gating ion channel

Ion channel

Ion channels are pore-forming proteins that help establish and control the small voltage gradient across the plasma membrane of cells by allowing the flow of ions down their electrochemical gradient. They are present in the membranes that surround all biological cells...

s directly.

For example, the G_βγ complex, when bound to histamine

Histamine

Histamine is an organic nitrogen compound involved in local immune responses as well as regulating physiological function in the gut and acting as a neurotransmitter. Histamine triggers the inflammatory response. As part of an immune response to foreign pathogens, histamine is produced by...

 receptors, can activate phospholipase

Phospholipase

A phospholipase is an enzyme that hydrolyzes phospholipids into fatty acids and other lipophilic substances. There are four major classes, termed A, B, C and D, distinguished by the type of reaction which they catalyze:*Phospholipase A...

 A₂. G_βγ complexes bound to muscarinic

Muscarine

Muscarine, L--muscarine, or muscarin is a natural product found in certain mushrooms, particularly in Inocybe and Clitocybe species, such as the deadly C. dealbata. Mushrooms in the genera Entoloma and Mycena have also been found to contain levels of muscarine which can be dangerous if ingested...

 acetylcholine

Acetylcholine

The chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system and central nervous system in many organisms including humans...

 receptors, on the other hand, directly open G protein-coupled inward rectifying potassium channels

Inward-rectifier potassium ion channel

Inwardly rectifying potassium channels are a specific subset of potassium selective ion channels. To date, seven subfamilies have been identified in various mammalian cell types...

 (GIRKs). They can also activate L-type calcium channels, as in H₃ receptor pharmacology.