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

Area of Science:

• Plant Biology
• Molecular Biology
• Biochemistry

Background:

• Brassinosteroids (BRs) are vital steroidal plant hormones regulating growth, development, and stress responses.
• BR signaling initiates with the export of bioactive BRs to the apoplast for receptor binding.
• ABCB19, an ATP-binding cassette (ABC) transporter, was previously identified as a BR exporter in Arabidopsis thaliana.

Purpose of the Study:

• To investigate the role of ABCB1, a homolog of ABCB19, in brassinosteroid transport.
• To elucidate the mechanism of BR export mediated by ABC transporters.

Main Methods:

• Biochemical assays to measure ATP hydrolysis activity of ABCB1 stimulated by BRs.
• Transport assays using proteoliposomes and protoplasts to confirm ABCB1's transport function.
• X-ray crystallography to determine the structures of ABCB1 in different ligand-bound states.

Main Results:

• ABCB1 functions as a brassinosteroid transporter, with its activity stimulated by bioactive BRs.
• Structural determination revealed brassinolide (BL) binding within a hydrophobic cavity of ABCB1.
• BL binding induces conformational changes in ABCB1, providing mechanistic insights.

Conclusions:

• ABCB1 is confirmed as a brassinosteroid transporter, adding to the known mechanisms of BR export.
• The structural and biochemical data offer a deeper understanding of how ABC transporters facilitate BR transport across membranes.
• This research contributes to understanding plant hormone signaling and transport pathways.