Feedback regulation of adaptive immunity is a fundamental mechanism for controlling the overall output of different signal transduction pathways, including that mediated by the T-cell antigen receptor (TCR). Calcineurin and Ras are known to have essential functions during T-cell activation. However, how the calcineurin signalling pathway is terminated in the process is still largely unknown. Although several endogenous inhibitors of calcineurin have been reported, none fulfils the criteria of a feedback inhibitor, as their expression is not responsive to TCR signalling. Here we identify an endogenous inhibitor of calcineurin, named Carabin, which also inhibits the Ras signalling pathway through its intrinsic Ras GTPase-activating protein (GAP) activity. Expression of Carabin is upregulated on TCR signalling in a manner that is sensitive to inhibitors of calcineurin, indicating that Carabin constitutes part of a negative regulatory loop for the intracellular TCR signalling pathway. Knockdown of Carabin by short interfering RNA led to a significant enhancement of interleukin-2 production by antigen-specific T cells in vitro and in vivo. Thus, Carabin is a negative feedback inhibitor of the calcineurin signalling pathway that also mediates crosstalk between calcineurin and Ras.
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