Molecular basis for the substrate specificity of protein kinase B; comparison with MAPKAP kinase‐1 and p70 S6 kinase

The substrate specificity of protein kinase‐Bα (PKBα, also known as RAC kinase or Akt) was investigated using synthetic peptide substrates related to the sequence surrounding the phosphorylation site on glycogen synthase kinase‐3 (GSK3). The minimum sequence motif required for efficient phosphorylation was Arg‐Xaa‐Arg‐Yaa‐Zaa‐Ser/Thr‐Hyd, where Xaa is any amino acid, Yaa and Zaa are small residues other than glycine and Hyd is a bulky hydrophobic residue (Phe, Leu). The most effective substrate, Arg‐Pro‐Arg‐Thr‐Ser‐Ser‐Phe, was phosphorylated with a K m of 5 μM and V max of 260 U/mg. PKBα phosphorylated histone H2B ( K m 5 μM, V max 68 Ulmg) specifically at Ser‐36 which also lies in an Arg‐Xaa‐Arg‐Xaa‐Xaa‐Ser‐Hyd motif. The peptide Arg‐Pro‐Arg‐Ala‐Ala‐Thr‐Phe may be a relatively specific substrate for PKBα because, unlike other substrates, it is not phosphorylated by p70 S6 kinase or MAP kinase activated protein (MAPKAP) kinase‐1.