Structural insights into AtABCG25, an angiosperm-specific abscisic acid exporter

Cellular hormone homeostasis is essential for the precise spatial and temporal signaling responses and plant fitness. Abscisic acid (ABA) plays pivotal roles in orchestrating various developmental and stress responses and confers fitness benefits over ecological and evolutionary timescales in terrestrial Plants. Cellular ABA level is regulated by complex processes including biosynthesis, catabolism, and transport. AtABCG25 is the first identified ABA exporter through genetic screen which affects diverse ABA responses. Resolving the structure basis of ABCG25 in ABA exporting is critical for further manipulating ABA homeostasis and plant fitness. We utilized cryo-electron microscopy to elucidate the structural dynamics of AtABCG25, and successfully characterized different states including apo AtABCG25, ABA-bound AtABCG25 and ATP-bound AtABCG25 (E232Q). Notably, AtABCG25 forms a homodimer, featuring a deep, slit-like cavity in the transmembrane domain. The critical residues in the cavity where ABA binds are precisely characterized. Moreover, ATP binding triggers the closure of nucleotide-binding domains and conformational transitions in the transmembrane domains. We show that AtABCG25 belongs to a conserved ABCG subfamily that originated only during the evolution of angiosperms. They neofunctionalized to regulate seed germination from endosperm, concerted to this angiosperm-specific innovation of embryo nourishing tissue. Collectively, these findings provide valuable insights into the intricate substrate recognition and transport mechanisms of ABA exporter AtABCG25, paving the way towards genetical manipulating of ABA homeostasis and plant fitness.

ABC transporters; AtABCG25; abscisic acid; cryo-EM; structure.