Enzymatic characteristics of a Ser/Thr protein kinase, SpkA, from Myxococcus xanthus

Two Ser/Thr protein kinases, SpkA and SpkB, selected from Myxococcus xanthus based on amino acid sequence similarities with the catalytic subunits of cAMP-dependent protein kinases (PKA) were synthesized using a cell-free protein synthesis system. In various protein kinase assays, purified StkA and StkB showed their highest protein kinase activities in a PKA assay using the selective PKA substrate Kemptide and in a protein kinase C (PKC) assay using the selective PKC substrate neurogranin(28–43), respectively. SpkA had apparent Km values of 45 μM and 37 μM for Kemptide and ATP, respectively. Phosphorylation of Kemptide was inhibited by a specific PKA inhibitor peptide, PKI5–24, and the IC50 and Ki values for inhibition of the SpkA activity were 117 nM and 36 nM, respectively.