Lyn and Fgr are activated in distinct membrane fractions of human granulocytic cells

We have previously shown that the src-family protein-tyrosine kinase Hck is localized on the azurophil granules of human granulocytes, translocates towards the phagosomes during phagocytosis of opsonized zymosan and is activated during this process. Hck is also activated upon cell stimulation with the calcium ionophore A23187, but not with PMA or the chemotactic peptide fMLP. Here, we investigated whether the src-family kinases Lyn and Fgr are activated under the same conditions. Upon stimulation of human neutrophils or retinoic acid-differentiated NB4 cells with fMLP, only Lyn is activated. Cell stimulation with opsonized zymosan or A23187 leads to simultaneous activation of Lyn and Fgr. In cell fractionation experiments with differentiated NB4 cells, the kinases show a similar subcellular localization: Both co-fractionate quantitatively with plasma-membrane marker in two fractions that sediment at 11000 g and 200000 g. Lyn is selectively activated in the 200000 g fraction, whereas Fgr is activated in the 11000 g fraction and distinct sets of tyrosine phosphorylated proteins are found in these fractions. The results suggest that these kinases exert their functional roles in distinct cellular compartments in human granulocytic cells.