Why are G protein only found in Eukaryote cell ?

they bind to protein-coupled transmembrane receptors with higher complexity than those found in prokaryotes

Explanation:

G-proteins are proteins found inside the cells that function as molecular switches which are activated by binding to guanosine triphosphate (GTP), while they are inactive by binding to guanosine diphosphate (GDP). The G-proteins bind to G-protein-coupled transmembrane receptors (GPCRs) in the cytoplasmic region. The GPCRs are a very diverse group of proteins that are activated by extracellular molecules ranging from small peptides to large proteins, including pheromones, neurotransmitters, light-sensitive compounds, etc, thereby allowing them to respond to diverse stimuli from the extracellular environment. In consequence, it is reasonable to suppose that the signaling pathways in which G proteins are involved have a higher complexity level than those observed in primitive prokaryotic organisms.