Multiple signal transduction pathways induce phosphorylation of serines 16, 25, and 38 of oncoprotein 18 in T lymphocytes.
A multitude of external signals induce extensive phosphorylation of Oncoprotein 18 (Op18), which suggests a putative role for this protein in signal transduction. We have recently identified two distinct proline-directed kinase families that phosphorylates Op18 with overlapping but distinct site preference. These two kinase families, mitogen-activated protein (MAP) kinases and cyclin-dependent cdc2 kinases, are involved in receptor- and cell cycle-regulated phosphorylation events, respectively. In the present study, site-specific phosphorylation of Op18 in response to stimulation of the antigen receptor-associated CD3 complex was analyzed in the Jurkat T cell-line. The results show that CD3-induced phosphorylation of Ser-25 of Op18, which is the primary MAP kinase phosphorylation site, can be induced by an apparently protein kinase C (PKC)-independent signal transduction pathway. We also demonstrate that Ser-16 of Op18 is specifically phosphorylated in response to the Ca2+ signal generated by CD3 stimulation or by the Ca2+ ionophore ionomycin. Ser-16 phosphorylation occurs independently of both PKC and MAP kinase activation. Using site-specific Op18 mutants and tryptic phosphopeptide mapping, we show that phosphorylation of Ser-16 of Op18 together with Ser-25, or Ser-25 and Ser-38, generates two Op18 phosphoisomers showing a dramatic electrophoretic retardation. In conclusion, site-mapping studies of Op18 reveal that CD3 stimulation results in an apparently PKC-independent activation of both the MAP kinase and a Ca(2+)-regulated kinase pathway, which results in phosphorylation of distinct sites of Op18. The data also pinpoints the specific phosphorylation events that result in electrophoretic retardation of Op18.