Activation of heterotrimeric G-proteins is accomplished exclusively by the action of GPCRs, transmembrane spanning receptors that typically reside in the plasma membrane. G-proteins consist of α, β and γ subunits, each involved in signaling to distinct effectors. The Gβγ dimer binds to a hydrophobic pocket present in Gα-GDP. GTP binding to Gα removes the hydrophobic pocket and reduces the affinity of Gα for Gβγ. Subsequently Gα-GTP dissociates from Gβγ and from the receptor. Both Gα-GTP and Gβγ are then free to activate downstream effectors.Gβγ-mediated signaling occurs through interactions with a variety of molecules including enzymes, ion channels and small G-proteins. The effectors directly regulated by Gβγ include PLC, potassium channels, AC and PI3K. Although each of these effectors exists as multiple isoforms, only specific isoforms are regulated by Gβγ. PLC is one of the major effectors of Gβγ. Gβγ also activates PLC-γ indirectly through the activation of BTK. The pleckstrin homology domain of BTK interacts with Gβγ leading to increased kinase activity and activation of PLCγ. Activation of mammalian PI-PLC results in the hydrolysis of PIP2 to release the second messengers DAG and IP3. IP3 diffuses through the cytosol and releases stored Ca2+ ions from the ER. DAG activates PKC, which in turn leads to phosphorylation of various proteins that are involved in an array of cellular events. PKC activates the ERK cascade including direct phosphorylation of either MEK or Raf1.
Another important substrate of Gβγ is PI3K. PI3K phosphorylates PIP2 to produce PIP3 at the inner leaflet of the plasma membrane. The binding of PIP3 to the PH domain anchors AKT to the plasma membrane and allows its phosphorylation and activation by PDK1. Activated AKT then phosphorylates several substrates and contributes to cell survival and cell proliferation. Gβγ can also regulate cAMP production by activating or inhibiting different AC isoforms. However, Gβγ subunits of the pertussis toxin-sensitive Gαi can enhance the effect of Gαs on type II and IV AC, but can inhibit Gαs stimulated type I AC. The other three Gαs stimulated AC (type III, V and VI) are not modulated by Gβγ subunits. Activation of EGFR leads to activation of the Ras/Raf/MAPK pathway. One major signaling role for Gβγis the management of MAPK cascades mediated through Src. Src activates ERKs via Ras. Src can trigger SHC phosphorylation and subsequent activation of the Ras exchanger SOS. Src can also activate Ras through transactivation of the EGF receptor and related receptors following stimulation of GPCRs via Gβγ subunits and the assembly of multiprotein complexes of dynamin-2 and caveolin-1 at clathrin dependent sites of endocytosis. Gβγ also regulates N-type Ca2+ channels. Gβγ subunits also directly bind to and activate a class of K+ channels called GIRKs. Gβγ binds PAK1 which activates CDC42. The Gβγ/PAK1/PIXα/CDC42 pathway is essential for the localization of F-actin formation to the leading edge, the exclusion of PTEN from the leading edge, directional sensing, and the persistent directional migration of chemotactic leukocytes.
G-proteins regulate important cellular components, such as metabolic enzymes, ion channels, and the transcriptional machinery. This results in the propagation of regulated activities through increasingly complex layers of organization.