Activation and interaction with protein kinase C of a cytoplasmic tyrosine kinase, Itk/Tsk/Emt, on FcεRI cross-linking on mast cells

Cross-linking of the high-affinity IgE receptor (FcεRI) on mast cells induces rapid phosphorylation on serine, threonine, and tyrosine residues and increases the enzymatic activity, of a Tec subfamily tyrosine kinase, Itk/Tsk/Emt (Emt). The pleckstrin homology domain of Emt at its amino- terminal interacts directly with multiple isoforms of protein kinase C (PKC) in vitro. In addition, a portion of Emt is physically associated with multiple isoforms of PKC in intact mast cells. PKC phosphorylates a bacterial fusion protein containing the pleckstrin homology domain of Emt in vitro. Coexpression of Emt in COS-7 cells with Ca²⁺-dependent PKC isoforms (α, βI, or βII) induces an enhancement in tyrosine phosphorylation of Emt. In vivo inhibition of PKC expression or activity attenuates tyrosine phosphorylation and enzymatic activity of Emt induced upon FcεRI cross- linking. These data collectively suggest that PKC pbosphorylates Emt and activates its autophosphorylating activity. Alternatively, PKC could activate another tyrosine kinase that phosphorylates Emt, or PKC-mediated phosphorylation of Emt may render it a target for another tyrosine kinase. In any case, PKC appears to play a major role in the activation of Emt induced upon FcεRI cross-linking.