InterPro: IPR001632 G-protein, beta subunit

Guanine nucleotide binding proteins (G proteins) are membrane-associated, heterotrimeric proteins composed of three subunits: alpha (IPR001019), beta (IPR001632) and gamma (IPR001770) [1]. G proteins and their receptors (GPCRs) form one of the most prevalent signalling systems in mammalian cells, regulating systems as diverse as sensory perception, cell growth and hormonal regulation [2]. At the cell surface, the binding of ligands such as hormones and neurotransmitters to a GPCR activates the receptor by causing a conformational change, which in turn activates the bound G protein on the intracellular-side of the membrane. The activated receptor promotes the exchange of bound GDP for GTP on the G protein alpha subunit. GTP binding changes the conformation of switch regions within the alpha subunit, which allows the bound trimeric G protein (inactive) to be released from the receptor, and to dissociate into active alpha subunit (GTP-bound) and beta/gamma dimer. The alpha subunit and the beta/gamma dimer go on to activate distinct downstream effectors, such as adenylyl cyclase, phosphodiesterases, phospholipase C, and ion channels. These effectors in turn regulate the intracellular concentrations of secondary messengers, such as cAMP, diacylglycerol, sodium or calcium cations, which ultimately lead to a physiological response, usually via the downstream regulation of gene transcription. The cycle is completed by the hydrolysis of alpha subunit-bound GTP to GDP, resulting in the re-association of the alpha and beta/gamma subunits and their binding to the receptor, which terminates the signal [3]. The length of the G protein signal is controlled by the duration of the GTP-bound alpha subunit, which can be regulated by RGS (regulator of G protein signalling) proteins (IPR000342) or by covalent modifications [4].

There are several isoforms of each subunit, many of which have splice variants, which together can make up hundreds of combinations of G proteins. The specific combination of subunits in heterotrimeric G proteins affects not only which receptor it can bind to, but also which downstream target is affected, providing the means to target specific physiological processes in response to specific external stimuli [5, 6]. G proteins carry lipid modifications on one or more of their subunits to target them to the plasma membrane and to contribute to protein interactions.

This entry consists of the G protein beta subunit, which assumes a barrel-shaped beta-propeller structure containing WD-40 repeats preceded by an N-terminal alpha helix. The beta subunit forms a stable dimer with the gamma subunit. The alpha subunit only contacts the beta subunit in the dimer, lying on the opposite face from the gamma subunit. RGS proteins that contain GGL (G protein gamma-like) domains can interact with beta subunits to form novel dimers that prevent gamma subunit binding, and may prevent heterotrimer formation by inhibiting alpha subunit binding.