GTP-binding proteins (G-proteins) play a critical role in signal transduction by coupling receptors to effectors. The intracellular consequences of G-protein activation include second messenger generation, protein phosphorylation, ion channel activation, gene induction, cell growth, and differentiation. Gi belongs to the pertussis toxin sensitive G-proteins. Gi consists of three different subtypes of alpha-subunits (alphai1, alphai2 and alphai3).