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The catalytic subunits of cAMP-dependent protein kinases (protein kinase A) from bovine heart and Ascaris suum muscle exhibit only 48% sequence identity and show quantitative differences in substrate specificity. These differences were not obvious at the level of short synthetic substrate peptides but were distinct for some protein substrates. Phosphofructokinase from Ascaris, a physiological substrate, was a better substrate for the protein kinase from the nematode in comparison to the mammalian protein kinase due to a 10-fold lower Michaelis constant. Selective phosphorylation by the two kinases was also observed with some in vitro substrates. In addition, quantitative differences in the interactions between R- and C-subunits from Ascaris and bovine heart were observed. However, several synthetic peptides whose sequence reflected the phosphorylation site of Ascaris suum phosphofructokinase (AKGRSDS*IV), or variations of it, were phosphorylated with the same efficiency by both protein kinases. Based on the data the following are concluded: (1) In agreement with the conservation of structure in the catalytic cleft, the recognition of substrates by protein kinases from phylogenetically distant organisms exhibits similarity. (2) Non-conserved parts of the surface of the protein kinase molecule may contribute to binding of protein substrates and thus to selective recognition.

Comparison of the Substrate Specificities of cAMP-Dependent Protein Kinase from Bovine Heart and Ascaris suum Muscle