Role of phosphorylated aminoacyl residues in generating atypical consensus sequences which are recognized by casein kinase-2 but not by casein kinase-1

Casein kinase-2 (CK-2) is a ubiquitous Ser/Thr specific protein kinase that recognizes phosphorylatable residues located upstream of acidic determinants, its consensus sequence being Ser(Thr)-Xaa-Xaa-Acidic. Here we show that the phosphotetrapeptide AcSer(P)-Ser(P)-Ser-Ser(P), which is devoid of the canonical consensus sequence, is nevertheless phosphorylated by CK-2 with rates comparable to that of typical peptide substrates Ser-Glu-Glu-Glu-Glu-Glu and Arg-Arg-Arg-Glu-Glu-Glu-Thr-Glu-Glu-Glu routinely employed for assaying CK-2 activity. The phosphopeptide AcSer(P)-Ser-Ser(P) [but not Ac-Ser-Ser(P)-Ser(P) or AcSer(P)-Ser(P)-Ser] is also phosphorylated albeit less efficiently than AcSer(P)-Ser(P)-Ser-Ser(P). Further N-terminal elongation with additional phosphoseryl residues to give the peptides AcSer(P)-Ser(P)-Ser(P)-Ser-Ser(P) and AcSer(P)-Ser(P)-Ser(P)-Ser(P)-Ser-Ser(P) does not improve but rather slightly decreases the phosphorylation efficiency by CK-2. These two peptides are conversely excellent substrates for CK-1, which does not appreciably phosphorylate either AcSer(P)-Ser-Ser(P) or AcSer-(P)-Ser(P)-Ser-Ser(P). Either individual or multiple replacement of the phosphorylated residues with glutamic acid in the peptide AcSer(P)-Ser(P)-Ser-Ser(P) drastically reduces the phosphorylation efficiency by CK-2, the phosphoseryl residue at position -2 playing an especially crucial role which cannot be surrogated by glutamyl residues.(ABSTRACT TRUNCATED AT 250 WORDS)