Some protein kinases are known to be activated by D-erythro-sphingosine (Sph) or N,N-dimethyl-D-erythrosphingosine (DMS), but not by ceramide, Sph-1-P, other sphingolipids, or phospholipids. Among these, a specific protein kinase that phosphorylates Ser 60 , Ser 59 , or Ser 58 of 14-3-3␤, 14-3-3, or 14-3-3, respectively, was termed "sphingosine-dependent protein kinase-1" (SDK1) (Megidish, T., Cooper, J., Zhang, L., Fu, H., and Hakomori, S. (1998) J. Biol. Chem. 273, 21834 -21845). We have now

more »

... dentified SDK1 as a protein having the C-terminal half kinase domain of protein kinase C␦ (PKC␦) based on the following observations. (i) Large-scale preparation and purification of proteins showing SDK1 activity from rat liver (by six steps of chromatography) gave a final fraction with an enhanced level of an ϳ40-kDa protein band. This fraction had SDK1 activity ϳ50,000-fold higher than that in the initial extract. (ii) This protein had ϳ53% sequence identity to the Ser/Thr kinase domain of PKC␦ based on peptide mapping using liquid chromatography/mass spectrometry and liquid chromatography/tandem mass spectrometry data. (iii) A search for amino acid homology based on the BLAST algorithm indicated that the only protein with high homology to the ϳ40-kDa band is the kinase domain of PKC␦. The kinase activity of PKC␦ did not depend on Sph or DMS; rather, it was inhibited by these sphingoid bases, i.e. PKC␦ did not display any SDK1 activity. However, strong SDK1 activity became detectable when PKC␦ was incubated with caspase-3, which releases the ϳ40-kDa kinase domain. PKC␦ and SDK1 showed different lipid requirements and substrate specificity, although both kinase activities were inhibited by common PKC inhibitors. The high susceptibility of SDK1 to Sph and DMS accounts for their important modulatory role in signal transduction.