Preference of human cdc2 kinase for peptide substrate.

: Human cyclin B1-bound cdc2 kinase phosphorylated the threonine residue in the sequence -Thr-Pro-Lys-Lys-Ala- but hardly phosphorylated it in the sequence -Thr-Pro-Lys-Ala-Lys. The sequence -Thr-Pro-Ala-Pro-Lys-, as found in p53 protein, was also phosphorylated by this enzyme, but less efficiently than in the sequence described above. When the threonine residue in -Thr-Pro-Lys-Lys-Ala- was changed to a serine or a tyrosine residue, the enzyme phosphorylated the serine, but not the tyrosine residue. Changing the lysine next to the proline to alanine reduced its efficiency as a substrate. The peptide, Ala-Ala-Ala-Ala-Lys-Thr-Pro-Ala-Lys-Ala-Ala, containing the -Thr-Pro-Ala-Lys- sequence, but not the other lysine residues, was not used as a substrate by the kinase.