Adrenergic receptors are members of the G protein family (see section 1.4.3). They possess seven regions of 20 to 24 hydrophobic amino acids crossing the membrane in α-helixes [89]. Their amino acid homologies are strongest for these membrane spanning domains, followed by the first two cytoplasmic loops. The third cytoplasmic loop (CIII) and the amino and carboxyl termini are most divergent within the adrenoceptor family. The amino terminal region contains two sites for asparagine-linked glycosylation. With the help of deleted mutants of the β2 adrenergic receptor [90, 91] it has been shown that glycosylation deficient mutants do not change their agonist binding characteristics [92, 93] nor in the signal transduction, as binding to the G protein and stimulating the adenylate cyclase (see page §), whereas the density of receptors is significantly reduced. The glycosylation therefore plays a major role in the translocation of the receptor to the plasma membrane. The binding site for agonists and antagonists results from interaction of the catecholamine group with the carboxylic group of Asp-113 in the 3rd hydrophobic domain of the β 2 receptor [94, 95, 96, 97, 98]. Agonistic effects are induced by hydrogen bonds of the hydroxy groups of Ser-204 and Ser-207 and the meta- or para-hydroxy groups of the ligand.
For stabilising the binding box, cysteines in extracellular loops are essential. When Cys-106 and Cys-184 are substituted for valine, the ligand binding strength is dramatically reduced [84, 77]. Amino acid substitutions of the membrane domains MI, MII, MIII and MVII, which represent highly conserved regions, also decrease the ligand binding affinity. After glycosylation, the β2 adrenergic receptor is also modified by palmitoylisation at Cys-341 (N-terminal domain). This posttranslational modification is, as shown by deleted mutants [99], crucial for the interaction with the G protein. The functions of the first two cytoplasmic loops are less well characterised.
The gene of the human β2 adrenergic receptor has been localised to chromosome 5q31–q32 (Table 1.5) [100]. The gene is intronless throughout [101]. Its promoter region contains a variety of binding sites for regulatory elements, including a steroid receptor binding hexamer (TGTTCT) [101, 102], cyclic AMP responsive elements [103], a consensus TATA box and a consensus CAAT box [101]. Interestingly, a non-overlapping cistron in the β2 receptor mRNA 5’ leader region is translated and the resulting peptide inhibits receptor translation [104]. Various alleles of the β2 adrenergic receptor gene have been found, amongst them, a single base substitution at position 1309 leading to an amino acid exchange (amino acid 16) from arginine to glycine.
There was shown by LIGGETT [105], that substitution of an extracellular cysteine enhances receptor phosphorylation and desensitisation. He [106] and others [107, 108] have shown, using site-directed mutagenesis, that mutations involving small regions of the β2 adrenergic receptor, including changes of a single amino acid, can markedly alter the functional properties of the receptor.
The β2 adrenoceptor gene seems to be one of the candidate genes for essential hypertension, on which researchers will focus their investigations in the coming years. There is evidence that β2 receptor density in cultured fibroblasts correlates with human salt sensitivity, which appears to be an important factor involved in the pathogenesis of essential hypertension [109]. Receptor density itself has been correlated with the SNP at position 1309 [110].
The β2 adrenergic receptor cDNA was sequenced by several groups [111, 112, 113]. The derived sequence consists of 413 amino acids1.
© 2001 Alexander Binder