Effect of multiple serine/alanine mutations in the transmembrane spanning region V of the D2 dopamine receptor on ligand binding

Three conserved serine residues (Ser193, Ser194, and Ser197) in transmembrane spanning region (TM) V of the D2 dopamine receptor have been mutated to alanine, individually and in combination, to explore their role in ligand binding and G protein coupling. The multiple Ser -->Ala mutations had no effect on the binding of most antagonists tested, including 3H]spiperone, suggesting that the multiple mutations did not affect the overall conformation of the receptor protein. Double or triple mutants containing an Ala197 mutation showed a decrease in affinity for domperidone, whereas Ala193 mutants showed an increased affinity for a substituted benzamide, remoxipride. However, dopamine showed large decreases in affinity (>20-fold) for each multiple mutant receptor containing the Ser193Ala mutation, and the high-affinity (coupled) state of the receptor (in the absence of GTP) could not be detected for any of the multiple mutants. A series of monohydroxylated phenylethylamines and aminotetralins was tested for their binding to the native and multiple mutant D2 dopamine receptors. The results obtained suggest that Ser193 interacts with the hydroxyl of S-5-hydroxy-2-dipropylaminotetralin (OH-DPAT) and Ser197 with the hydroxyl of R-5-OH-DPAT. We predict that Ser193 interacts with the hydroxyl of R-7-OH-DPAT and the 3-hydroxyl (m-hydroxyl) of dopamine. Therefore, the conserved serine residues in TMV of the D2 dopamine receptor are involved in hydrogen bonding interactions with selected antagonists and most agonists tested and also enable agonists to stabilise receptor-G protein coupling.