Cellular responses leading to development, proliferation, and differentiation depend on RAF/MEK/ERK signaling, which integrates and amplifies signals from various stimuli for downstream cellular responses. C-RAF activation has been reported in many types of tumor cell proliferation and developmental disorders, necessitating the discovery of potential C-RAF protein regulators. Here, we identify a novel and specific protein interaction between C-RAF among the RAF kinase paralogs, and SIRT4 among the mitochondrial sirtuin family members SIRT3, SIRT4, and SIRT5. Structurally, C-RAF binds to SIRT4 through the N-terminal cysteine-rich domain, whereas SIRT4 predominantly requires the C-terminus for full interaction with C-RAF. Interestingly, SIRT4 specifically interacts with C-RAF in a pre-signaling inactive (serine 259-phosphorylated) state. Consistent with this finding, the expression of SIRT4 in HEK293 cells results in an up-regulation of pS259-C-RAF levels and a concomitant reduction in MAPK signaling as evidenced by strongly decreased phospho-ERK signals. Thus, we propose an additional extra-mitochondrial function of SIRT4 as a cytosolic tumor suppressor of C-RAF-MAPK signaling, besides its metabolic tumor suppressor role of glutamate dehydrogenase and glutamate levels in mitochondria.

中文翻译：

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SIRT4 作为 MAPK 信号传导中 C-RAF 激酶的新型相互作用因子和候选抑制剂。

导致发育、增殖和分化的细胞反应依赖于 RAF/MEK/ERK 信号传导，它整合并放大来自下游细胞反应的各种刺激的信号。据报道，许多类型的肿瘤细胞增殖和发育障碍中都有 C-RAF 激活，因此需要发现潜在的 C-RAF 蛋白调节剂。在这里，我们鉴定了 RAF 激酶旁系同源物中的 C-RAF 与线粒体 Sirtuin 家族成员 SIRT3、SIRT4 和 SIRT5 中的 SIRT4 之间的新颖且特异性的蛋白质相互作用。在结构上，C-RAF 通过 N 端富含半胱氨酸的结构域与 SIRT4 结合，而 SIRT4 主要需要 C 端才能与 C-RAF 完全相互作用。有趣的是，SIRT4 在前信号传导失活（丝氨酸 259 磷酸化）状态下与 C-RAF 特异性相互作用。与这一发现一致的是，HEK293 细胞中 SIRT4 的表达导致 pS259-C-RAF 水平上调，同时 MAPK 信号传导减少，磷酸-ERK 信号强烈减少证明了这一点。因此，我们提出，除了其谷氨酸脱氢酶和线粒体中谷氨酸水平的代谢肿瘤抑制作用之外，SIRT4还具有作为C-RAF-MAPK信号传导的胞质肿瘤抑制因子的额外线粒体外功能。