How does stimulation of GPCRs activate G-protein subunits?
A.
When an extracellular signal molecule binds to a GPCR, the receptor protein undergoes a conformational change that enables it to activate a G protein located on the other (cytosolic) side of the plasma membrane.
B.
The alteration of the α subunit of the G protein allows it to exchange its GDP for GTP.
C.
This exchange triggers an additional conformational change that activates both the α subunit and a βγ complex, which dissociate to interact with their preferred target proteins in the plasma membrane.
D.
The receptor stays active as long as the external signal molecule is bound to it, and it can therefore activate many molecules of G protein.