Structural basis of auxin recognition and transport in plant influx carrier AUX1

Abstract

Auxin regulates many aspects of plant growth and development, featuring polar auxin transport mediated by auxin efflux and influx carriers. AUX1 is the prominent auxin importer that actively takes up natural and synthetic auxins. However, the precise mechanisms by which AUX1 recognizes and transports auxin remain elusive. Here, we describe the cryo-electron microscopy structures of Arabidopsis thaliana AUX1 in apo and auxin-bound forms, revealing the structural basis for auxin recognition. AUX1 assumes the LeuT-like fold in an inward conformation. The auxin analogue 2,4-D is recognized by polar residues in the middle of AUX1. We identify a putative cation site in AUX1, which plays a role in stabilizing the inward-facing conformation. His249 undergoes a large conformational shift, and mutation of it completely abolished transport activity, suggesting a crucial role of His249 in AUX1 gating. Together, this study provides a structural foundation for a deeper comprehension of auxin influx by AUX1-like carriers.