Dopamine D2 Receptors Retain Agonist High-Affinity Form and Guanine Nucleotide Sensitivity after Removal of Sialic Acid

Abstract

The ligand binding subunit of the D₂ dopamine receptor (Mr_??_94, 000) can be visualized by autoradiography following photoaffinity labeling with [¹²⁵I] N-azidophenethylspiperone and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Following removal of sialic acids with the exoglycosidase, neuraminidase, [¹²⁵I]N-azidophenethylspiperone photoincorporated into a protein of Mr=54, 000 with the appropriate pharmacological profile for D₂ receptors. The desialylated D₂ receptor bound dopaminergic agonists with high affinity and was capable of coupling to a functional G-protein as indexed by: 1) pertussis-toxin mediated [³²P] ADP ribosylation of proteins of Mr=42, 000 and 39, 000, and 2) the conversion of the agonist high affinity form of D₂ receptors to one displaying low affinity for agonists in the presence of guanine nucleotides. These data suggest that sialic acid residues do not contribute significantly to the ligand binding characteristics of D₂ receptors despite the large change produced in the estimated molecular mass of the binding subunit.