Non-receptor tyrosine kinase that plays a redundant role with ITK in regulation of the adaptive immune response. Regulates the development, function and differentiation of conventional T-cells and nonconventional NKT-cells. When antigen presenting cells (APC) activate T-cell receptor (TCR), a series of phosphorylation leads to the recruitment of TXK to the cell membrane, where it is phosphorylated at Tyr-420. Phosphorylation leads to TXK full activation. Contributes also to signaling from many receptors and participates in multiple downstream pathways, including regulation of the actin cytoskeleton. Like ITK, can phosphorylate PLCG1, leading to its localization in lipid rafts and activation, followed by subsequent cleavage of its substrates. In turn, the endoplasmic reticulum releases calcium in the cytoplasm and the nuclear activator of activated T-cells (NFAT) translocates into the nucleus to perform its transcriptional duty. Plays a role in the positive regulation of IFNG transcription in T-helper 1 cells as part of an IFNG promoter-binding complex with PARP1 and EEF1A1 (By similarity). Within the complex, phosphorylates both PARP1 and EEF1A1 (By similarity). Phosphorylates also key sites in LCP2 leading to the up-regulation of Th1 preferred cytokine IL-2. Phosphorylates 'Tyr-201' of CTLA4 which leads to the association of PI-3 kinase with the CTLA4 receptor. ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] Activated by phosphorylation by FYN. Interacts with PARP1 and EEF1A1 (By similarity). Interacts with SH2D2A (PubMed:10587356). Interacts with FYN (PubMed:11353545). Localizes in the vicinity of cell surface receptors in the plasma membrane after receptor stimulation. Translocates into the nucleus and enhances IFN-gamma gene transcription in T-cells. Expressed in early thymocytes, T-cells and mast cells. Phosphorylated at Tyr-420 by FYN. Autophosphorylation at Tyr-91 is critical for the activation of TXK, leading to the up-regulation of IFN-gamma gene transcription. The cysteine string at the N-terminus is palmitoylated and required for the proper subcellular location. Produced by alternative initiation at Met-55 of isoform 1. Belongs to the protein kinase superfamily. Tyr protein kinase family. TEC subfamily. Unlike the other TEC subfamily members, TXK is activated independently of the activity of phosphatidylinositol 3-kinase, consistent with its lack of a PH domain. Membrane association is performed through palmitoylation at the N-terminus.