CHARACTERIZATION OF AN ACTIVE, NON-MYRISTYLATED, CYTOPLASMIC FORM OF THE LYMPHOID PROTEIN TYROSINE KINASE PP56LCK

pp56lck is a member of the src family of tyrosine kinases mainly expressed in T lymphocytes. Src tyrosine kinases have been implicated in the control of cell growth and differentiation in different cell types, but the mechanism of regulation of these enzymes is poorly understood. In order to characterize the distinct species of pp56lck, we have produced high yields of enzymatically active wild type pp56lck using the eukaryotic baculovirus expression system in Spodoptera frugiperda insect cells (Sf9). We find that the various species of baculoviral pp56lck are not only differentially phosphorylated (on serine and tyrosine residues) but also heterogeneously myristylated. Surprisingly a non-myristylated, very active form of bv-pp56lck is found in the cytoplasm of Sf9 cells. Fractionation of T cells reveals that cytoplasmic pp56lck exists in T lymphocytes as well.