The first transmembrane segment of the dopamine D2 receptor: Accessibilityin the binding-site crevice and position in the transmembrane bundle

The binding site of the dopamine D2 receptor, like that of homologous G-pro tein-coupled receptors (GPCRs), is contained within a water-accessible crev ice formed among its seven transmembrane segments (TMs). Using the substitu ted-cysteine-accessibility method (SCAM), we are mapping the residues that contribute to the surface of this binding-site crevice. We have now mutated to cysteine, one at a time, 21 consecutive residues in TM1. Six of these m utants reacted with charged sulfhydryl reagents, whereas bound antagonist o nly protected N52(1.50)C from reaction. Except for A38(1.36)C, none of the substituted cysteine mutants in the extracellular half of TM1 appeared to b e accessible. Pro(1.48) is highly conserved in opsins, but absent in catech olamine receptors, and the high-resolution rhodopsin structure showed that Pro(1.48) bends the extracellular portion of TMI inward toward TM2 and TM7. Analysis of the conversation of residues in the extracellular portion of T MI of opsins showed a pattern consistent with a-helical structure with a co nserved face. In contrast, this region in catecholamine receptors is poorly conserved, suggesting a lack of critical contacts. Thus, in catecholamine receptors in the absence of Pro(1.48), TMI may be straighter and therefore further from the helix bundle, consistent with the apparent lack of conserv ed contact residues. When examined in the context of a model of the D2 rece ptor, the accessible residues in the cytoplasmic half of TMI are at the int erface with TM7 and with helix 8 (H8). We propose the existence of critical contacts of TM1, TM7, and HS that may stabilize the inactive state of the receptor.