Summary The transcription factor NFAT plays key roles in multiple biological activities, such as immune responses, tissue development, and malignant transformation. NFAT is dephosphorylated by calcineurin, which is activated by intracellular calcium levels, and translocated into the nucleus, resulting in transcriptional activation. Calcineurin dephosphorylates various target proteins and regulates their functions. However, the regulation of NFAT degradation is largely unknown, and it is unclear whether calcineurin contributes to the stability of NFAT. We investigated the effect of calcineurin inhibition on NFAT protein stability, and found that the dephosphorylation of NFAT by calcineurin promotes the NFAT stabilization, whereas calcineurin mutant that is defective in phosphatase activity was unable to stabilize NFAT. Increased intracellular calcium ion concentration, which is essential for calcineurin activation, also induced NFAT stability. In addition, we identified S-phase kinase associated protein 2 (Skp2), an F-box protein of the SCF ubiquitin ligase complex, as a factor mediating degradation of NFAT when calcineurin was depleted. In summary, these findings revealed that the dephosphorylation of NFAT by calcineurin protects NFAT from degradation by Skp2 and promotes its protein stability.

中文翻译：

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钙调磷酸酶对 NFAT 的去磷酸化抑制 Skp2 介导的降解

摘要 转录因子 NFAT 在免疫反应、组织发育和恶性转化等多种生物活动中发挥着关键作用。NFAT 被钙调神经磷酸酶去磷酸化，钙调神经磷酸酶被细胞内钙水平激活，并易位到细胞核中，导致转录激活。钙调神经磷酸酶使各种靶蛋白去磷酸化并调节其功能。然而，NFAT 降解的调控很大程度上是未知的，并且不清楚钙调神经磷酸酶是否有助于 NFAT 的稳定性。我们研究了钙调磷酸酶抑制对 NFAT 蛋白稳定性的影响，发现钙调磷酸酶对 NFAT 的去磷酸化促进了 NFAT 的稳定，而磷酸酶活性缺陷的钙调磷酸酶突变体无法稳定 NFAT。细胞内钙离子浓度的增加（这对于钙调神经磷酸酶的激活至关重要）也诱导了 NFAT 的稳定性。此外，我们还发现 S 期激酶相关蛋白 2 (Skp2)（SCF 泛素连接酶复合物的 F 盒蛋白）是钙调神经磷酸酶耗尽时介导 NFAT 降解的因子。总之，这些发现表明，钙调神经磷酸酶对 NFAT 的去磷酸化可保护 NFAT 免受 Skp2 的降解，并促进其蛋白质稳定性。