A specific protein kinase that phosphorylates Ser60, Ser59, or Ser58 of 14-3-3beta, eta, zeta, respectively, only in the presence of sphingosine (Sph) or N,N-dimethyl-Sph (DMS), was termed "sphingosine-dependent protein kinase-1" (SDK1) [J. Biol. Chem. 273(34) (1998) 21834]. We have now identified SDK1 as a protein having the same amino acid sequence as in the C-terminal-half kinase domain of PKCdelta. with similar to40kDa. molecular mass, based on large-scale purification of a protein from rat liver, and partial sequence using three different combinations of LC-MS or LC-MS/MS with respective search engine. PKCdelta did not display any SDK1 activity and PKCdelta activity was inhibited by Sph and DMS. However, strong SDK1 activity, only in the presence of Sph or DMS. became detectable when PKCdelta was incubated with caspase-3, which releases the similar to40 kDa kinase domain. (C) 2003 Elsevier Science (USA). All rights reserved.