Agonist versus antagonist action of ATP at the P2Y4 receptor is determined by the second extracellular loop.
UTP is a potent full agonist at both the human P2Y(4) (hP2Y(4)) and rat P2Y(4) (rP2Y(4)) receptor. In contrast, ATP is a potent full agonist at the rP2Y(4) receptor but is a similarly potent competitive antagonist at the hP2Y(4) receptor. To delineate the structural determinants of agonism versus antagonism in these species homologues, we expressed a series of human/rat P2Y(4) receptor chimeras in 1321N1 human astrocytoma cells and assessed the capacity of ATP and UTP to mobilize intracellular Ca(2+). Replacement of the NH(2) terminus of the hP2Y(4) receptor with the corresponding region of the rP2Y(4) receptor resulted in a receptor that was activated weakly by ATP, whereas replacement of the second extracellular loop (EL2) of the hP2Y(4) receptor with that of the rP2Y(4) receptor yielded a chimeric receptor that was activated fully by UTP and near fully by ATP, albeit with lower potencies than those observed at the rP2Y(4) receptor. These potencies were increased, and ATP was converted to a full agonist by replacing both the NH(2) terminus and EL2 in the hP2Y(4) receptor with the corresponding regions from the rP2Y(4) receptor. Mutational analysis of the five divergent amino acids in EL2 between the two receptors revealed that three amino acids, Asn-177, Ile-183, and Leu-190, contribute to the capacity of EL2 to impart ATP agonism. Taken together, these results suggest that the second extracellular loop and the NH(2) terminus form a functional motif that plays a key role in determining whether ATP functions as an agonist or antagonist at mammalian P2Y(4) receptors.
Herold
Christopher L
CL
Department of Pharmacology, University of North Carolina, Chapel Hill, North Carolina 27599-7365, USA.
Qi
Ai-Dong
AD
Harden
T Kendall
TK
Nicholas
Robert A
RA
D000255:Adenosine Triphosphate / Q000494:pharmacology*; D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002460:Cell Line; D006801:Humans; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D058910:Purinergic P2 Receptor Agonists; D058919:Purinergic P2 Receptor Antagonists; D051381:Rats; D018048:Receptors, Purinergic P2 / Q000737:chemistry / Q000187:drug effects*; D011993:Recombinant Fusion Proteins / Q000819:agonists / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000187:drug effects; D017386:Sequence Homology, Amino Acid
D058910:Purinergic P2 Receptor Agonists; D058919:Purinergic P2 Receptor Antagonists; D018048:Receptors, Purinergic P2; D011993:Recombinant Fusion Proteins; C104749:purinoceptor P2Y4; D000255:Adenosine Triphosphate