Dougherty Michele K, Müller Jürgen, Ritt Daniel A, Zhou Ming, Zhou Xiao Zhen, Copeland Terry D, Conrads Thomas P, Veenstra Timothy D, Lu Kun Ping, Morrison Deborah K
Laboratory of Protein Dynamics and Signaling, National Cancer Institute-Frederick, Frederick, MD 21702, USA.
Mol Cell. 2005 Jan 21;17(2):215-24. doi: 10.1016/j.molcel.2004.11.055.
The Raf-1 kinase is an important signaling molecule, functioning in the Ras pathway to transmit mitogenic, differentiative, and oncogenic signals to the downstream kinases MEK and ERK. Because of its integral role in cell signaling, Raf-1 activity must be precisely controlled. Previous studies have shown that phosphorylation is required for Raf-1 activation, and here, we identify six phosphorylation sites that contribute to the downregulation of Raf-1 after mitogen stimulation. Five of the identified sites are proline-directed targets of activated ERK, and phosphorylation of all six sites requires MEK signaling, indicating a negative feedback mechanism. Hyperphosphorylation of these six sites inhibits the Ras/Raf-1 interaction and desensitizes Raf-1 to additional stimuli. The hyperphosphorylated/desensitized Raf-1 is subsequently dephosphorylated and returned to a signaling-competent state through interactions with the protein phosphatase PP2A and the prolyl isomerase Pin1. These findings elucidate a critical Raf-1 regulatory mechanism that contributes to the sensitive, temporal modulation of Ras signaling.
Raf-1激酶是一种重要的信号分子，在Ras信号通路中发挥作用，将促有丝分裂、分化和致癌信号传递给下游激酶MEK和ERK。由于其在细胞信号传导中的不可或缺的作用，Raf-1的活性必须受到精确控制。先前的研究表明，磷酸化是Raf-1激活所必需的，在此，我们鉴定出六个磷酸化位点，这些位点在有丝分裂原刺激后对Raf-1的下调起作用。所鉴定的位点中有五个是活化ERK的脯氨酸定向靶点，并且所有六个位点的磷酸化都需要MEK信号传导，这表明存在一种负反馈机制。这六个位点的过度磷酸化会抑制Ras/Raf-1相互作用，并使Raf-1对其他刺激不敏感。随后，过度磷酸化/不敏感的Raf-1通过与蛋白磷酸酶PP2A和脯氨酰异构酶Pin1相互作用而被去磷酸化，并恢复到具有信号传导能力的状态。这些发现阐明了一种关键的Raf-1调节机制，该机制有助于对Ras信号进行敏感的、适时的调节。
