Determination of the Specific Substrate Sequence Motifs of Protein Kinase C Isozymes*

Protein kinase C (PKC) family members play significant roles in a variety of intracellular signal transduction processes, but information about the substrate specificities of each PKC family member is quite limited. In this study, we have determined the optimal peptide substrate sequence for each of nine human PKC isozymes (α, βI, βII, γ, δ, ε, η, μ, and ζ) by using an oriented peptide library. All PKC isozymes preferentially phosphorylated peptides with hydrophobic amino acids at position +1 carboxyl-terminal of the phosphorylated Ser and basic residues at position −3. All isozymes, except PKCμ, selected peptides with basic amino acids at positions −6, −4, and −2. PKCα, -βI, -βII, -γ, and -η selected peptides with basic amino acid at positions +2, +3, and +4, but PKCδ, -ε, -ζ, and -μ preferred peptides with hydrophobic amino acid at these positions. At position −5, the selectivity was quite different among the various isozymes; PKCα, -γ, and -δ selected peptides with Arg at this position while other PKC isozymes selected hydrophobic amino acids such as Phe, Leu, or Val. Interestingly, PKCμ showed extreme selectivity for peptides with Leu at this position. The predicted optimal sequences from position −3 to +2 for PKCα, -βI, -βII, -γ, -δ, and -η were very similar to the endogenous pseudosubstrate sequences of these PKC isozymes, indicating that these core regions may be important to the binding of corresponding substrate peptides. Synthetic peptides based on the predicted optimal sequences for PKCα, -βI, -δ, -ζ, and -μ were prepared and used for the determination of Km and Vmax for these isozymes. As judged by Vmax/Km values, these peptides were in general better substrates of the corresponding isozymes than those of the other PKC isozymes, supporting the idea that individual PKC isozymes have distinct optimal substrates. The structural basis for the selectivity of PKC isozymes is discussed based on residues predicted to form the catalytic cleft.

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