Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 118, Issue 33, Pages -Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.2101838118
Keywords
brassinosteroid; signal transduction; BZR1; turnover; nucleocytoplasmic trafficking
Categories
Funding
- NSF of China [31970313, 91417313, 31201063]
- Department of Education of Hebei Province [LJRC015]
- Natural Science Foundation of Hebei Province, China [C2019205288]
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BZR1, a key transcription factor in the brassinosteroid signaling pathway, is tightly regulated by phosphorylation and subcellular localization; it can be rapidly recruited from the cytoplasm to the nucleus by BR treatment, and its protein abundance is regulated by two distinct turnover mechanisms.
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.
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