A synthetic peptide substrate specific for casein kinase I.

The synthetic peptide, Asp-Asp-Asp-Glu-Glu-Ser-Ile-Thr-Arg-Arg, derived from the phosphorylation site of casein kinase-1 (CK-1) in β-casein A2, is readily phosphorylated by CK-1, but not by casein kinase-2 (CK-2), cyclic AMP-dependent protein kinase, protein kinsae C, phosphorylase kinase and protein kinase Fa. Phosphorylation by CK-1 occurs only at Ser-6, Thr-8 being unaffected. The Km for the peptide is higher (1 mM) than for β-casein A2 (40 μM), while the Vmaxis quite comparable. This is the first synthetic peptide substrate for CK-1 described so far, and can be used for the rapid and specific estimation of CK-1 activity in crude extracts. © 1989.