ACTIVATION OF PHOSPHATIDYLINOSITOL-3-KINASE IN JURKAT T-CELLS DEPENDSON THE PRESENCE OF THE P56(LCK) TYROSINE KINASE

Activation of resting T lymphocytes by ligands to the T cell receptor (TcR)/CD3 complex is initiated by phosphorylation of a number of key r egulatory proteins on specific tyrosine residues. One such protein is the heterodimeric enzyme phosphatidylinositol-3-kinase (PI3K). We rece ntly found that this enzyme is also rapidly activated following TcR/CD 3 triggering and that immunoprecipitated PI3K was activated in vitro b y direct tyrosine phosphorylation. Here we show that TcR/CD3-induced t yrosine phosphorylation and activation of PI3K in Jurkat T leukemia ce lls depend on the presence of the p56(lck) tyrosine kinase: in a varia nt of the Jurkat T cell line lacking p56(lck), JCaM1, these responses were absent. We also show that p56(lck) directly activates PI3K purifi ed from transfected COS-1 cells, indicating that other T cell-specific proteins are not required for the process. Finally, tryptic peptide m aps show that p56(lck) phosphorylates three tyrosine residues in the p 85 alpha subunit of PI3K and two in p110 of PI3K. Our results suggest that p56(lck) is required for activation of PI3K in Jurkat T cells and can itself directly activate it by phosphorylating one or several sti mulatory sites.