Beta-adrenergic receptor kinase. Agonist-dependent receptor binding promotes kinase activation

The beta-adrenergic receptor kinase (beta ARK) specifically phosphorylates the activated form of multiple receptors such as the beta 2-adrenergic receptor (beta 2 AR) and rhodopsin. beta ARK also phosphorylates synthetic peptides, albeit with an approximately 10(4)-10(7)-fold lower Vmax/Km ratio as compared to receptors, with a clear preference for peptides containing acidic residues on the aminoterminal side of a serine or threonine. To further characterize the mechanism of substrate phosphorylation by beta ARK, we designed a series of analogue peptides containing a single amino acid change (serine, glutamic acid, or phosphoserine) situated 2 or 4 residues amino-terminal to the target serine. While beta ARK weakly phosphorylated peptides lacking an acidic residue, peptides containing either a single phosphoserine or glutamic acid were substantially better substrates with a 3.5- to 8-fold increase in Vmax. Additional studies demonstrated that the interaction of beta ARK with an activated receptor (beta 2AR* or Rho*) also significantly enhanced peptide phosphorylation. Both Rho* and a truncated rhodopsin lacking its carboxyl-terminal phosphorylation sites activated peptide phosphorylation to a similar extent with EC50 values for activation of 0.65 and 1.34 microM, respectively. In contrast, the agonist-occupied beta 2AR activated peptide phosphorylation by beta ARK with a substantially higher affinity (EC50 of 0.012 microM) compared to Rho*. The Vmax/Km ratio for beta ARK phosphorylation of a poor peptide substrate such as RRRASAAASAA was increased up to approximately 200-fold by the activated receptor while the phosphorylation of a good peptide substrate (RRREEEEESAAA) was increased only up to approximately 8-fold. Our results suggest that acidic residues (glutamic acid or phosphoserine) localized on the amino-terminal side of target serines are important but not essential determinants in directing peptide phosphorylation. The substrate specificity of beta ARK appears to rely more strongly on the overall topological structure of the activated receptor which promotes the specific binding and activation of beta ARK.