Emt/Itk associates with activated TCR complexes: role of the pleckstrin homology domain.

Expressed in mast and T-cells/inducible T cell tyrosine kinase (Emt/Itk) is a protein tyrosine kinase required for T cell Ag receptor (TCR)-induced activation and development. A physical interaction between Emt/Itk and TCR has not been described previously. Here, we have utilized laser scanning confocal microscopy to demonstrate that Ab-mediated engagement of the CD3epsilon chain induces the membrane colocalization of Emt/Itk with TCR/CD3. Removal of the Emt/Itk pleckstrin homology domain (DeltaPH-Emt/Itk) abrogates the association of the kinase with the cell membrane, as well as its activation-induced colocalization with the TCR complex and subsequent tyrosine phosphorylation. The addition of a membrane localization sequence to DeltaPH-Emt/Itk from Lck restores all of these deficiencies except the activation-induced tyrosine phosphorylation. Our data suggest that the PH domain of Emt/Itk can be replaced with another membrane localization signal without affecting the membrane targeting and activation-induced colocalization of the kinase with the TCR. However, the PH domain is indispensable for the activation-induced tyrosine phosphorylation of the kinase.