Structural features determining the site specificity of a rat liver cAMP-independent protein kinase.

The Seryl and Threonyl residues affected in α_s1 and in β-caseins by rat liver “casein kinase TS” (a cytosolic cAMP-independent protein kinase) have been identified. All of them, as well as the residues affected by the same enzyme in α_s2-casein are characterized by an acidic group two residues to their C terminus and by being located within predicted β-turns. Several other potential sites of phosphorylation, according to their primary structure, but located outside predicted β-turns, are not significantly labeled by the protein kinase. It seems conceivable therefore that both a definite aminoacid sequence including a critical acidic residue, and the existence of a β-turn are required for the activity of this protein kinase.