Gi protein alpha subunit is a family of heterotrimeric G proteinalpha subunits. This family is also commonly called the Gi/o (Gi /Go ) family or Gi/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 activate intracellular 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 a heterotrimeric 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 a Gβγ complex.[1][2] When not stimulated by a receptor, Gα is bound to GDP and to Gβγ to form the inactive G protein trimer.[1][2] When the receptor binds an activating ligand outside the cell (such as a hormone or neurotransmitter), the activated receptor acts as a guanine nucleotide exchange factor to promote GDP release from and GTP 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 receive light during vision. Rhodopsin in dim light night vision in retinal rod cells couples to Gt1, and color photopsins in color vision in retinal cone cells couple to Gt2, respectively. Gt3/Gustducin subunits transduce signals in the sense of taste (gustation) in taste buds by coupling to G protein-coupled receptors activated by sweet or bitter substances.
