Abstract

Computer simulations on a new model of the α1b-adrenergic receptor based on the crystal structure of rhodopsin have been combined with experimental mutagenesis to investigate the role of residues in the cytosolic half of helix 6 in receptor activation. Our results support the hypothesis that a salt bridge between the highly conserved arginine (R143^3.50) of the E/DRY motif of helix 3 and a conserved glutamate (E289^6.30) on helix 6 constrains the α1b-AR in the inactive state. In fact, mutations of E289^6.30 that weakened the R143^3.50-E289^6.30 interaction constitutively activated the receptor. The functional effect of mutating other amino acids on helix 6 (F286^6.27, A292^6.33, L296^6.37, V299^6.40,V300^6.41, and F303^6.44) correlates with the extent of their interaction with helix 3 and in particular with R143^3.50 of the E/DRY sequence.