Gi protein alpha subunit is a family ofheterotrimeric G proteinalpha subunits. This family is also commonly called theGi/o (Gi/Go) family orGi/o/z/t family to include closely related family members. G alpha subunits may be referred to as Gi alpha, Gαi, or Giα.
The general function of Gi/o/z/t is to activateintracellular signaling pathways in response to activation of cell surface G protein-coupled receptors (GPCRs). GPCRs function as part of a three-component system of receptor-transducer-effector.[1][2] The transducer in this system is aheterotrimeric G protein, composed of three subunits: a Gα protein such as Giα, and a complex of two tightly linked proteins called Gβ and Gγ in aGβγ complex.[1][2] When not stimulated by a receptor, Gα is bound toGDP and to Gβγ to form the inactive G protein trimer.[1][2] When the receptor binds an activating ligand outside the cell (such as ahormone orneurotransmitter), the activated receptor acts as aguanine nucleotide exchange factor to promote GDP release from andGTP binding to Gα, which drives dissociation of GTP-bound Gα from Gβγ.[1][2] GTP-bound Gα and Gβγ are then freed to activate their respective downstream signaling enzymes.
Gz proteins also can link GPCRs to inhibition of adenylyl cyclase, but Gz is distinct from Gi/Go by being insensitive to inhibition by pertussis toxin.[5]
Gt proteins function in sensory transduction. The Transducins Gt1 and Gt2 serve to transduce signals from G protein-coupled receptors that receivelight duringvision.Rhodopsin in dim lightnight vision in retinalrod cells couples to Gt1, and colorphotopsins in color vision in retinalcone cells couple to Gt2, respectively. Gt3/Gustducin subunits transduce signals in the sense oftaste (gustation) intaste buds by coupling to G protein-coupled receptors activated by sweet or bitter substances.
