Interleukin 2-induced protein phosphorylation/dephosphorylation in human non-adherent peripheral blood mononuclear cells: initial phosphorylation of two 75-kDa proteins.

The kinetics of protein phosphorylation/dephosphorylation induced by IL-2 in cells of the non-adherent subpopulation of human peripheral blood mononuclear cells after pretreatment with and without phytohaemagglutinin (PHA) was analyzed by two-dimensional gel electrophoresis. In cells not pretreated with PHA, IL-2 induced the phosphorylation of two 75-kDa proteins with pI values 6.6 and 6.9, detectable 30 min after addition of IL-2, and the dephosphorylation of a 94-kDa (pI 4.0) protein, two 85-kDa (pI 5.2 and 5.4) proteins and a 65-kDa (pI 4.9) protein. The latter three phosphoproteins were found to be unlabelled after PHA pretreatment, but upon IL-2 stimulation the 94-kDa and the 85-kDa proteins became labelled simultaneously with two minor phosphoproteins of 68 kDa (pI 5.7) and 37 kDa (pI 4.8). Moreover, PHA pretreatment of cells induced a drastic phosphorylation of a 48-kDa (pI 6.5) and a 46-kDa (pI 6.7) protein, which were gradually dephosphorylated after IL-2 addition. Phosphorylation of the 75-kDa proteins could not be detected when the cells were pretreated with PHA prior to labelling. These results suggest that IL-2-induced phosphorylation of the 75-kDa proteins is one of the early events in IL-2 stimulation, an event already completed in PHA-pretreated cells since PHA is known to induce release of IL-2. Furthermore, the retarded appearance of the labelled 75-kDa proteins suggests an IL-2-induced de novo synthesis, possibly reflecting the expression of the 75-kDa alpha chain of the IL-2 receptor.