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.
There are four distinct subtypes of alpha subunits in the Gi/o/z/t alpha subunit family that define four families of heterotrimeric G proteins:
Gi proteins: Gi1α, Gi2α, and Gi3α
Go protein: Goα (in mouse there is alternative splicing to generate Go1α and Go2α)
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.
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.