When a ligand activates the G protein-coupled receptor, it induces a conformational change in the receptor that allows the receptor to function as a guanine nucleotide exchange factor (GEF) that exchanges GDP for GTP. The GTP (or GDP) is bound to the G subunit in the traditional view of heterotrimeric GPCR activation. This exchange triggers the dissociation of the G subunit (which is bound to GTP) from the G dimer and the receptor as a whole. However, models which suggest molecular rearrangement, reorganization, and pre-complexing of effector molecules are beginning to be accepted. Both G-GTP and G can then activate different signaling cascades (or second messenger pathways) and effector proteins, while the receptor is able to activate the next G protein.