A Cytoplasmic Negative Regulator Isoform of ATF7 Impairs ATF7 and ATF2 Phosphorylation and Transcriptional Activity

Alternative splicing and post-translational modifications are processes that give rise to the complexity of the proteome. The nuclear ATF7 and ATF2 (activating transcription factor) are structurally homologous leucine zipper transcription factors encoded by distinct genes. Stress and growth factors activate ATF2 and ATF7 mainly via sequential phosphorylation of two conserved threonine residues in their activation domain. Distinct protein kinases, among which mitogen-activated protein kinases (MAPK), phosphorylate ATF2 and ATF7 first on Thr71/Thr53 and next on Thr69/Thr51 residues respectively, resulting in transcriptional activation. Here, we identify and characterize a cytoplasmic alternatively spliced isoform of ATF7. This variant, named ATF7-4, inhibits both ATF2 and ATF7 transcriptional activities by impairing the first phosphorylation event on Thr71/Thr53 residues. ATF7-4 indeed sequesters the Thr53-phosphorylating kinase in the cytoplasm. Upon stimulus-induced phosphorylation, ATF7-4 is poly-ubiquitinated and degraded, enabling the release of the kinase and ATF7/ATF2 activation. Our data therefore conclusively establish that ATF7-4 is an important cytoplasmic negative regulator of ATF7 and ATF2 transcription factors.

可变剪接与翻译后修饰是造就蛋白质组复杂性的关键生物学过程。核内ATF7与ATF2（激活转录因子，activating transcription factor）是由不同基因编码、结构同源的亮氨酸拉链型转录因子。应激与生长因子主要通过对二者激活结构域内两个保守苏氨酸残基的顺序磷酸化，激活ATF2与ATF7。不同的蛋白激酶中以丝裂原活化蛋白激酶（MAPK）为代表，可分别先在Thr71/Thr53位点、后在Thr69/Thr51位点磷酸化ATF2与ATF7，进而介导转录激活。本研究鉴定并表征了ATF7的一种胞质可变剪接异构体。该变异体命名为ATF7-4，可通过削弱Thr71/Thr53位点的首次磷酸化过程，同时抑制ATF2与ATF7的转录活性。ATF7-4可在胞质中螯合负责磷酸化Thr53位点的蛋白激酶。当受到刺激诱导发生磷酸化时，ATF7-4会发生多泛素化并被降解，从而释放蛋白激酶，使ATF7与ATF2得以激活。因此，本研究数据确凿证实，ATF7-4是ATF7与ATF2转录因子的重要胞质负调控因子。