Phosphopeptide substrates of a phosphoprotein phosphatase from rat liver

The substrate specificity of a preparation of phosphoprotein phosphatase (Mr = 32 000) from rat liver was investigated. Phosphopeptides based on the structure Leu-Arg-Arg-Ala-Ser(P)-Val-Ala-Glx-Leu and Ala-Arg-Thr-Lys-Arg-Ser-Gly-Ser(P)-Val-Tyr-Glu-Pro-Leu-Lys were used. These phosphopeptides correspond to the phosphorylation sites of rat liver pyruvate kinase (type L) and the beta subunit of rabbit muscle phosphorylase b kinase, respectively. A decrease in the apparent Km values and a concomitant increase in Vmax values was observed when the number of amino acyl residues after the phosphoseryl residue in the respective phosphopeptides were increased from 2 to 4, 5, or 6. Most of the phosphopeptides investigated generally showed apparent Km values higher than the values obtained with phosphopyruvate kinase. Ala-Ser(P)-Val-Ala and Gly-Ser(P)-Val-Tyr appeared to be the shortest phosphopeptides that could be dephosphorylated rapidly. These findings support the hypothesis that a small part of the phosphoprotein may be sufficient to fulfill the minimal requirements for its dephosphorylation.