This is one of the several different receptors for 5-hydroxytryptamine (serotonin), a biogenic hormone that functions as a neurotransmitter, a hormone, and a mitogen. The activity of this receptor is mediated by G proteins that stimulate adenylate cyclase. Isoform 5-HT4(A) interacts with MAGI2, MPP3, SLC9A3R1 and SNX27 isoforms 1 and 2. Isoform 5-HT4(E) interacts with PATJ, NOS1 and SEC23A. Isoform 5-HT4(A) forms a complex including SLC9A3R1 and EZR (By similarity). Interacts (via C-terminus 330-346 AA) with GRK5; this interaction is promoted by 5-HT (serotonin). Interaction with SNX27 mediates recruitment to early endosomes, while interaction with SLC9A3R1 and EZR might target the protein to specialized subcellular regions, such as microvilli. Additional isoforms seem to exist. Isoform 5-HT4(A) is expressed in ileum, brain, and atrium, but not in the ventricle. Mainly expressed in atria and cardiac ventricle. Expressed in all cardiovascular tissues analyzed. Belongs to the G-protein coupled receptor 1 family.