Gαq binds two effectors separately in cells:: Evidence for predetermined signaling pathways

G-proteins transduce signals along diverse pathways, but the factors involved in pathway selection are largely unknown. Here, we have studied the ability of G alpha(q) to select between two effectors-mammalian inositide-specific phospholipase C beta(PLC beta) and phosphoinositide-3-kinase (PI3K)-in human embryonic kidney 293 cells. These studies were carried out by measuring interactions between eCFP- and eYFP-tagged proteins using Forster resonance energy transfer in the basal state and during stimulation. Instead of association of G alpha(q) with effectors through diffusion and exchange, we found separate and stable pools of G alpha(q)-PLC beta and G alpha(q)-PI3K complexes existing throughout the stimulation cycle. These separate complexes existed despite the ability of G alpha(q) to simultaneously bind both effectors as determined by in vitro measurements using purified proteins. Preformed G-protein/effector complexes will limit the number of pathways that a given signal will take, which may simplify predictive models.