The Ste20 kinase TAOK3 restrains Rac-driven cytoskeletal-mitochondrial coupling to preserve naive CD8+ T cell homeostasis and activation.

Cytoskeletal remodelling is central to naive T cell fitness, organizing receptor-proximal signaling and mechanotransduction during TCR engagement. However, how cytoskeletal dynamics are coordinated with TCR signaling to preserve naive T cell fitness remains incompletely defined. Here, we identify the Sterile 20-family member Thousand and One Kinase 3 (TAOK3) as a kinase-dependent regulator of naive CD8+ T cell maintenance that couples TCR signal integration to cytoskeletal control. Genetic deletion or kinase inactivation of TAOK3 resulted in a profound, cell-intrinsic loss of naive CD8+ T cells. Despite enhanced sensitivity to TCR ligation and enhanced downstream signaling, proliferating CD8+ T cells did not survive in vitro and anti-viral CD8+ T cell immunity was compromised in vivo in the absence of TAOK3. Unbiased phospho-proteomic analysis of Taok3-deficient mice revealed altered phosphorylation of the Rac regulators Dedicator of Cytokinesis DOCK8 and DOCK10, alongside actin-membrane scaffolding proteins. Consistent with this, Taok3-deficient naive CD8+ T cells exhibited elevated basal actin polymerisation, excessive reactive oxygen species accumulation, mitochondrial hyperpolarisation, and reduced spare respiratory capacity. Pharmacologic Rac inhibition normalised cytoskeletal dynamics, corrected the heightened TCR sensitivity, and preferentially restored mitochondrial membrane potential. Collectively, these findings identify TAOK3 as a coordinator of membrane-proximal organisation and cytoskeletal regulation that calibrates Rac-dependent signaling, thereby linking TCR signal integration to mitochondrial fitness and long-term maintenance of the naive CD8+ T cell pool.