Ministry of Education Key Laboratory of Molecular and Cellular Biology, Hebei Collaboration Innovation Center for Cell Signaling and Environmental Adaptation, Hebei Key Laboratory of Molecular and Cellular Biology, College of Life Sciences, Hebei Normal University, 050024 Shijiazhuang, China.
Department of Bioscience and Bioengineering, Hebei University of Science and Technology, 050000 Shijiazhuang, China.
Proc Natl Acad Sci U S A. 2021 Aug 17;118(33). doi: 10.1073/pnas.2101838118.
Regulation of the nucleocytoplasmic trafficking of signaling components, especially transcription factors, is a key step of signal transduction in response to extracellular stimuli. In the brassinosteroid (BR) signal transduction pathway, transcription factors from the BRASSINAZOLE RESISTANT1 (BZR1) family are essential in mediating BR-regulated gene expression. The subcellular localization and transcriptional activity of BZR1 are tightly regulated by reversible protein phosphorylation; however, the underlying mechanism is not well understood. Here, we provide evidence that both BZR1 phosphorylation and dephosphorylation occur in the nucleus and that BR-regulated nuclear localization of BZR1 is independent from its interaction with, or dephosphorylation by, protein phosphatase 2A. Using a photoconvertible fluorescent protein, Kaede, as a living tag to distinguish newly synthesized BZR1 from existing BZR1, we demonstrated that BR treatment recruits cytosolic BZR1 to the nucleus, which could explain the fast responses of plants to BR. Additionally, we obtained evidence for two types of protein turnover mechanisms that regulate BZR1 abundance in plant cells: a BR- and 26S proteosome-independent constitutive degradation mechanism and a BR-activated 26S proteosome-dependent proteolytic mechanism. Finally, treating plant cells with inhibitors of 26S proteosome induces the nuclear localization and dephosphorylation of BZR1, even in the absence of BR signaling. Based on these results, we propose a model to explain how BR signaling regulates the nucleocytoplasmic trafficking and reversible phosphorylation of BZR1.
信号转导过程中，调控信号转导组分（尤其是转录因子）的核质转运是一个关键步骤。在油菜素内酯（BR）信号转导途径中，BZR1 家族的转录因子在介导 BR 调控的基因表达中起关键作用。BZR1 的亚细胞定位和转录活性受到可逆蛋白磷酸化的严格调控；然而，其潜在的机制尚不清楚。在这里，我们提供了证据表明，BZR1 的磷酸化和去磷酸化都发生在细胞核中，并且 BR 调控的 BZR1 核定位与它与蛋白磷酸酶 2A 的相互作用或去磷酸化无关。我们使用光可转化的荧光蛋白 Kaede 作为活标签，以区分新合成的 BZR1 和现有的 BZR1，结果表明 BR 处理将细胞质中的 BZR1 募集到细胞核中，这可以解释植物对 BR 的快速反应。此外，我们获得了两种调节植物细胞中 BZR1 丰度的蛋白周转机制的证据：一种是 BR 和 26S 蛋白酶体非依赖的组成性降解机制，另一种是 BR 激活的 26S 蛋白酶体依赖性蛋白水解机制。最后，用 26S 蛋白酶体抑制剂处理植物细胞，即使在没有 BR 信号的情况下，也能诱导 BZR1 的核定位和去磷酸化。基于这些结果，我们提出了一个模型来解释 BR 信号如何调节 BZR1 的核质转运和可逆磷酸化。
