Department of Biochemistry and Molecular Biology, New Jersey Medical School, University of Medicine and Dentistry of New Jersey, Newark, NJ 07103, USA.
Mol Cell. 2013 Jan 10;49(1):121-32. doi: 10.1016/j.molcel.2012.10.023. Epub 2012 Nov 29.
Human mitochondrial transcription factor A (TFAM) is a high-mobility group (HMG) protein at the nexus of mitochondrial DNA (mtDNA) replication, transcription, and inheritance. Little is known about the mechanisms underlying its posttranslational regulation. Here, we demonstrate that TFAM is phosphorylated within its HMG box 1 (HMG1) by cAMP-dependent protein kinase in mitochondria. HMG1 phosphorylation impairs the ability of TFAM to bind DNA and to activate transcription. We show that only DNA-free TFAM is degraded by the Lon protease, which is inhibited by the anticancer drug bortezomib. In cells with normal mtDNA levels, HMG1-phosphorylated TFAM is degraded by Lon. However, in cells with severe mtDNA deficits, nonphosphorylated TFAM is also degraded, as it is DNA free. Depleting Lon in these cells increases levels of TFAM and upregulates mtDNA content, albeit transiently. Phosphorylation and proteolysis thus provide mechanisms for rapid fine-tuning of TFAM function and abundance in mitochondria, which are crucial for maintaining and expressing mtDNA.
人线粒体转录因子 A（TFAM）是一种高迁移率族（HMG）蛋白，位于线粒体 DNA（mtDNA）复制、转录和遗传的交汇点。其翻译后调控的机制知之甚少。本文证明，TFAM 在其 HMG 盒 1（HMG1）内被 cAMP 依赖性蛋白激酶在线粒体中磷酸化。HMG1 磷酸化会损害 TFAM 与 DNA 结合和激活转录的能力。研究表明，只有无 DNA 的 TFAM 可被 Lon 蛋白酶降解，Lon 蛋白酶可被抗癌药物硼替佐米抑制。在 mtDNA 水平正常的细胞中，Lon 降解 HMG1 磷酸化的 TFAM。然而，在 mtDNA 严重缺乏的细胞中，非磷酸化的 TFAM 也被降解，因为它是无 DNA 的。在这些细胞中耗尽 Lon 会增加 TFAM 的水平并上调 mtDNA 含量，尽管是短暂的。磷酸化和蛋白水解为 TFAM 在线粒体中的功能和丰度的快速微调提供了机制，这对于维持和表达 mtDNA 至关重要。
