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G-protein coupled receptors, or GPCRs, are another type of integral membrane protein.

Their extracellular part protruding out of the membrane contains a ligand binding site for a variety of compounds. The intracellular portion is coupled to a G-protein made up of three different sub-units, alpha, beta, and gamma.

When a ligand binds, the receptor changes shape causing the guanosine diphosphate, or GDP, bound by the alpha subunit to be released and substituted for a guanosine triphosphate, or GTP, molecule. These two then dissociate from the beta and gamma subunits.

Now, both sets of fragments are free to move around the cytoplasm and interact with other effector proteins that activate a series of intracellular signal cascades that direct and amplify the signal. To terminate the process, GTP is hydrolyzed into GDP and the G-protein subunits reassemble back into an inactive form leaving the GPCR ready for a new signal.