In vivo phosphorylation of a synthetic peptide substrate of cyclic AMP-dependent protein kinase.

A model synthetic peptide substrate of the cyclic AMP-dependent protein kinase (ATP:protein phosphotransferase; EC 2.7.1.37), Leu-Arg-Arg-Ala-Ser-Leu-Gly, closely resembling the local phosphorylation site sequence in porcine hepatic pyruvate kinase, was shown to be phosphorylated in vivo after microinjection into Xenopus oocytes. This result demonstrates that the microinjection technique, utilizing a synthetic peptide substrate, or possibly a synthetic substrate analog inhibitor [Kemp, B. E., Benjamini, E. & Krebs, E. G. (1976) Proc. Natl. Acad. Sci. USA 73, 1038--1042], can be used to study protein phosphorylation-dephosphorylation reactions in living oocytes. This follows, since it is clear that the injected peptide was accessible to the cellular compartment containing the protein kinase.