The alpha subunit activates adenylate cyclase, in purple, and loses GTP. Adenylate cyclase converts ATP to cyclic AMP, which then activates Protein Kinase, shown in blue. Protein Kinase phosphorylates an ion channel, letting sodium ions rush into the cell. Notice that the alpha subunit, now carrying GDP, has reattached to the beta and gamma G protein subunits by the dopamine receptor.