Hold Them Back

Abstract

Brassinosteroids (BRs) are plant steroids. They bind to and activate the cell-surface receptor kinase BRI1, which initiates a cascade of responses that regulate the phosphorylation states of the transcription factors BZR1 and BZR2. Phosphorylation of BZRs renders them inactive, whereas dephosphorylation results in their activation. Gampala et al . investigated the mechanisms by which BR-mediated modulation of BZR1 phosphorylation affected its activity. They expressed a fusion protein of BZR1 and yellow fluorescent protein (YFP) in transgenic Arabidopsis and used confocal microscopy to show that treatment of seedlings with brassinolide (BL), a potent BR, resulted in the nuclear accumulation of BZR1-YFP. Subcellular fractionation and Western blotting demonstrated that nonphosphorylated BZR1 was enriched in the nucleus, whereas phosphorylated BZR1 was enriched in the cytosol. Radiolabeled DNA hybridized to nonphosphorylated but not phosphorylated recombinant BZR1 on a blot. A yeast two-hybrid screen identified two isoforms of 14-3-3 proteins as binding partners for BZR1. The 14-3-3 proteins bind to targets in a sequence- and phosphorylation-dependent manner and regulate many cellular processes, often by protein translocation (see commentary by de Vries). Recombinant 14-3-3λ bound to phosphorylated but not nonphosphorylated BZR1 protein on a blot. Transgenic Arabidopsis expressing BZR1Δ (which cannot bind 14-3-3) exhibited a phenotype consistent with constitutive activation of BZR1. BZR1Δ-YFP showed increased nuclear localization compared with that of BZR1-YFP. Treatment of Arabidopsis with an inhibitor of 14-3-3 target protein interactions increased the nuclear accumulation of BZR1. Similar results were found for BZR2. Together, these data establish 14-3-3 proteins as important negative regulators of BR signaling. S. S. Gampala, T.-W. Kim, J.-X. He, W. Tang, Z. Deng, M.-Y. Bai, S. Guan, S. Lalonde, Y. Sun, J. M. Gendron, H. Chen, N. Shibagaki, R. J. Ferl, D. Ehrhardt, K. Chong, A. L. Burlingame, Z.-Y. Wang, An essential role for 14-3-3 proteins in brassinosteroid signal transduction in Arabidopsis . Dev. Cell 13 , 177-189 (2007). [PubMed] S. C. de Vries, 14-3-3 proteins in plant brassinosteroid signaling. Dev. Cell 13 , 162-164 (2007). [PubMed]