Key points are not available for this paper at this time.
Two tryptic phosphopeptides containing the sites on the alpha and beta subunits of phosphorylase kinase which are phosphorylated by protein kinase, dependent on adenosine 3':5'-monophosphate (cyclic AMP), have been isolated and their amino acid sequences have been determined. 32P-labelled phosphorylase kinase, containing 1.9 mol phosphate per mol enzyme, was digested with an equimolar quantity of trypsin for 2.5 min at pH 7.0, 20 degrees C. This treatment released nearly all the 32P radioactivity associated with the beta subunit as trichloroacetic-acid-soluble material. Only a small proportion of the 32P radioactivity associated with the alpha subunit was solubilised, the remainder being removed in the trichloroacetic acid pellet. The beta-subunit tryptic phosphopeptide was completely resolved from traces of the alpha-subunit phosphopeptide by gel filtration on Sephadex G-25. Further purification by peptide mapping separated the phosphopeptide into four components, each derived from the same nine-amino-acid segment of the betachain, which was found to possess the sequence: Gln-Ser-Gly-Ser(P)-Val-Ile-Tyr-Pro-Leu-Lys. The four components were produced by the partial cyclisation of the N-terminal glutaminyl residue, and by the presence of two alleles for the beta subunit in the rabbit population, which led to a valine-isoleucine ambiguity. The alpha-subunit phosphopeptide was liberated from the trichloroacetic acid pellet by redigestion with trypsin. It was the largest component in the digest which remained soluble in 5% trichloroacetic acid, and obtained in a highly purified form by a single filtration on Sephadex G-50. The peptide comprised 39 amino acids of which nine were serine and three were threonine residues. Only one residue, the serine at position three from the amino terminus, was phosphorylated. The amino-terminal sequence of the peptide was shown to be: Arg-Leu-Ser(P)-Ile-Ser-Thr-Glu-Ser-Glx-Pro-Asx-Gly. The sequences confirm the stoichiometry of the reaction and the absolute specificity of cyclic-AMP-dependent protein kinase for just two of the 200 serine residues in the enzyme. These results and an inspection of the rate of phosphorylation of a number of skeletal muscle proteins, including each enzyme of the glycolytic pathway, lead to the conclusion that cyclic-AMP-dependent protein kinase is an extremely specific enzyme. The molecular basis of this specificity is discussed.
Cohen et al. (Sat,) studied this question.