Characterization of a fluorescent substrate for the adenosine 3',5'-cyclic monophosphate-dependent protein kinase

A synthetic tetradecapeptide derived from the phosphorylation site of the beta-subunit of phosphorylase kinase (Arg-Thr-Lys-Arg-Ser-Gly-Ser-Val-Tyr-Glu-Pro-Leu-Lys-Ile) is a highly efficient substrate for the cAMP-dependent protein kinase, exhibiting a 36% decrease in the intrinsic tyrosine fluorescence on phosphorylation. The fluorescence changes in continuous assays were monitored to demonstrate the roles of protein kinase effectors (cAMP, the type II regulatory subunit, and the 8000-Da heat-stable inhibitor) in the regulation of the enzyme and to determine Km and Vmax. The phosphorylation reaction requires 1 mol ATP/mol peptide. Amino acid analysis demonstrates the presence of phosphoserine in the phosphorylated peptide. Auxiliary experiments show that tyrosine phosphorylation can also be detected fluorometrically and distinguished from serine or threonine phosphorylation.