Phosphorylation of the third intracellular loop of the mouse alpha(1b)-adrenergic receptor by cAMP-dependent protein kinase

The third intracellular loop of adrenergic receptors has been implicated in their interaction with guanine nucleotide-binding proteins (G proteins). One of the mechanisms involved in the modulation of receptor function is the phosphorylation of specific residues by intracellular kinases. alpha(1b)-Adrenergic receptor is phosphorylated in vitro by cAMP-dependent protein kinase (PKA), although its physiological effect remains to be determined. We have produced fusion proteins formed by glutathione S-transferase and sequences of the third intracellular loop of mouse alpha(1a)-, alpha(1b)-, and alpha(1d)-adrenergic receptor subtypes, and used them as substrates for PKA. Only the fusion protein containing the alpha(1b) sequence was phosphorylated in vitro by this kinase. Site-directed mutagenesis of a serine (homologue to serine 278 of the rat sequence, RSS) to an alanine residue precluded phosphorylation by PKA. (C) 1997 Elsevier Science Inc.