Translationally controlled tumor protein (TCTP) is a growth-related protein under transcriptional as well as translational control. We screened a rat skeletal muscle cDNA library using yeast two-hybrid system and found that TCTP interacts with the third large cytoplasmic domain of ␣1 as well as ␣2 isoforms of Na,K-ATPase, believed involved in the regulation of Na,K-ATPase activity. Interaction between TCTP and Na,K-ATPase was confirmed by coimmunoprecipitation in yeast and mammalian cells. We also showed, using 86 Rb ؉ uptake assay, that overexpression of TCTP inhibited Na,K-ATPase activity in HeLa cells. Northern and Western blotting studies of HeLa cells transiently transfected with GFPtagged TCTP showed that overexpression of TCTP did not change mRNA and protein levels of Na,K-ATPase. Recombinant TCTP protein purified from an Escherichia coli expression system inhibited purified HeLa cell plasma membrane Na,K-ATPase in a dose-dependent manner. Using deletion analysis, we also found that the C-terminal 102-172-amino-acid region of rat TCTP that contains the TCTP homology region 2 is essential for its association with, and inhibition of, Na,K-ATPase. Na,K-ATPase, a multimembrane-spanning enzyme, is essential for maintaining transmembrane gradients of Na ϩ and K ϩ ions and thus for cell homeostasis (1). These ionic gradients serve to control essential cellular processes such as cell volume, membrane potential, and nutrient transport (2). In addition, Na,K-ATPase is involved in cell proliferation and differentiation, heart and vascular muscle contraction, and neurotransmitter and hormone secretion (3). Thus dysfunction of this enzyme can profoundly affect cell function. Na,K-ATPase is composed of a catalytic 110-kDa ␣ subunit and a glycosylated 40 -60-kDa  subunit. The ␣ subunit contains binding sites for cations, ATP, and cardiac glycosides. It has been suggested that there might exist a diffusible cytoplasmic regulator of Na,K-ATPase activity, possibly modulated by protein kinases and hormones (4, 5). The third large cytoplasmic domain (CD3) 1 of Na,K-ATPase was proposed to be one of the domains involved in the regulation of its activity by insulin, thereby playing an important role in the catalytic function and regulation of this enzyme (6). Interactions between the Nterminal region of the Na,K-ATPase ␣ subunit with phosphoinositide-3 kinase (7), cytoplasmic domain 2 (CD2) and CD3 with ankyrin (8, 9), CD3 with cofilin (10), and purified Na,KATPase with actin (11) and adducin (12) have also been demonstrated.We looked for other cytoplasmic agents that might interact with the CD3 of Na,K-ATPase ␣ subunit and regulate its activity and found that translationally controlled tumor protein (TCTP) acts as a cytoplasmic repressor of Na,K-ATPase. TCTP is a growth-related protein, under tight transcriptional as well as translational control (13,14). It occurs as a 23-kDa protein in humans and has a 21-kDa homologue in mice but shows no significant homology with any other family of proteins. Based on structural studies o...