Itk tyrosine kinase substrate docking is mediated by a nonclassical SH2 domain surface of PLCγ1

Interleukin-2 tyrosine kinase (Itk) is a Tec family tyrosine kinase that mediates signaling processes after T cell receptor engagement. Activation of Itk requires recruitment to the membrane via its pleckstrin homology domain, phosphorylation of Itk by the Src kinase, Lck, and binding of Itk to the SLP-76/LAT adapter complex. After activation, Itk phosphorylates and activates phospholipase C-gamma 1 (PLC-gamma 1), leading to production of two second messengers, DAG and IP3. We have previously shown that phosphorylation of PLC-gamma 1 by Itk requires a direct, phosphotyrosine-independent interaction between the Src homology 2 (SH2) domain of PLC-gamma 1 and the kinase domain of Itk. We now define this docking interface using a combination of mutagenesis and NMRspectroscopy and show that disruption of the Itk/PLC gamma 1 docking interaction attenuates T cell signaling. The binding surface on PLC gamma 1 that mediates recognition by Itk highlights a nonclassical binding activity of the well-studied SH2 domain providing further evidence that SH2 domains participate in important signaling interactions beyond recognition of phosphotyrosine.