Brassinosteroids are steroidal phytohormones that regulate plant development and physiology, including adaptation to environmental stresses. Brassinosteroids are synthesized in the cell interior but bind receptors at the cell surface, necessitating a yet to be identified export mechanism. Here, we show that a member of the ATP-binding cassette (ABC) transporter superfamily, ABCB19, functions as a brassinosteroid exporter. We present its structure in both the substrate-unbound and the brassinosteroid-bound states. Bioactive brassinosteroids are potent activators of ABCB19 ATP hydrolysis activity, and transport assays showed that ABCB19 transports brassinosteroids. In Arabidopsis thaliana, ABCB19 and its close homolog, ABCB1, positively regulate brassinosteroid responses. Our results uncover an elusive export mechanism for bioactive brassinosteroids that is tightly coordinated with brassinosteroid signaling.

Center for Plant Systems Biology VIB 9052 Ghent Belgium

Department of Neurology of The 1st Affiliated Hospital of USTC MOE Key Laboratory for Membraneless Organelles and Cellular Dynamics Hefei National Research Center for Physical Sciences at the Microscale Biomedical Sciences and Health Laboratory of Anhui Province Division of Life Sciences and Medicine University of Science and Technology of China Hefei 230027 China

Department of Organic and Macromolecular Chemistry Ghent University 9000 Ghent Belgium

Department of Plant Biotechnology and Bioinformatics Ghent University 9052 Ghent Belgium

Institute of Science and Technology Austria 3400 Klosterneuburg Austria

Laboratory of Growth Regulators Institute of Experimental Botany The Czech Academy of Sciences and Palacký University 77900 Olomouc Czech Republic

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Structural insights into brassinosteroid export mediated by the Arabidopsis ABC transporter ABCB1