The two-pore domain K ؉ channel, TRESK (TWIK-related spinal cord K ؉ channel) is activated in response to the calcium signal by the calcium/calmodulin-dependent protein phosphatase, calcineurin. In the present study we report that calcineurin also interacts with TRESK via an NFAT-like docking site, in addition to its enzymatic action. In its intracellular loop, mouse TRESK possesses the amino acid sequence, PQIVID, which is similar to the calcineurin binding consensus motif, PXIXIT (where X denotes any amino acids), necessary for NFAT (nuclear factor of activated T cells) activation and nuclear translocation. Two-pore domain potassium (2PK ϩ ) 3 channels give rise to background (leak) potassium conductance and their diverse regulatory mechanisms control cellular function by adjusting both the resting membrane potential and excitability (for review, see Refs. 1 and 2). TRESK (TWIK-related spinal cord K ϩ channel) is uniquely regulated by the calcium signal among the 2PK ϩ channels. We have recently reported that TRESK, expressed heterologously in Xenopus oocytes, is activated about 10-fold by the calcium/calmodulin-dependent protein phosphatase, calcineurin (3). TRESK was cloned from human spinal cord (4) and mouse cerebellum (3), and its expression was also demonstrated by reverse transcription-PCR in cerebrum, brainstem, testis, pancreas, and placenta (5-7). A massive signal was detected by Northern blot in rat thymus and spleen (7). Single channel activity of TRESK has recently been demonstrated in dorsal root ganglion neurons (8). In the absence of specific antibodies and inhibitors, TRESK has neither been identified at the protein level nor has it been detected as an endogenous whole cell current. However, the robust activation of TRESK by the calcium signal implicates that it can influence substantially the function of the native cells expressing the channel. The calcineurin-mediated regulation suggests that TRESK activation is the target of the widely used immunosuppressive drugs, cyclosporine A and FK506, as it was, in fact, demonstrated in Xenopus oocytes (3). These drugs inhibit calcineurin (and consequently NFAT activation and interleukin-2 production of T lymphocytes) by forming inhibitory complexes with the ubiquitous immunophilin proteins (9, 10). However, general inhibition of calcineurin also causes several undesired effects. Therefore, a novel direction of drug development focuses on the NFATdocking site of calcineurin, which is considered to be a more specific target than the phosphatase activity (11,12). The binding of calcineurin to the PXIXIT consensus motif (where X denotes any amino acid) is required for NFAT activation. Apart from NFAT, the only known mammalian proteins possessing similar calcineurin binding sites are calcineurin inhibitors/modulators or anchoring proteins (13,14). In the present study we report for the first time that an ion channel, TRESK, has an NFAT-like calcineurin binding consensus sequence and the binding of the phosphatase to this docking site is indispensable for ...