Positive Regulation of Itk PH Domain Function by Soluble IP

Pleckstrin homology (PH) domain--mediated protein recruitment to cellular membranes is of paramount importance for signal transduction. The recruitment of many PH domains is controlled through production and turnover of their membrane ligand, phosphatidylinositol 3,4,5-trisphosphate (PIP₃). We show that phosphorylation of the second messenger inositol 1,4,5-trisphosphate (IP₃) into inositol 1,3,4,5-tetrakisphosphate (IP₄) establishes another mode of PH domain regulation through a soluble ligand. At physiological concentrations, IP₄ promoted PH domain binding to PIP₃. In primary mouse CD4⁺CD8⁺ thymocytes, this was required for full activation of the protein tyrosine kinase Itk after T cell receptor engagement. Our data suggest that IP₄ establishes a feedback loop of phospholipase C--γ1 activation through Itk that is essential for T cell development.