Arsenic-sensing domain controls ACR3 transporter trafficking in Marchantia polymorpha

Arsenic is a toxic, carcinogenic metalloid that threatens human health through contaminated water and food. Arsenite [As(III)] efflux mediated by ACR3 transporters is an ancient detoxification mechanism conserved across prokaryotes, fungi, algae, and non-angiosperm land plants, yet plant ACR3 proteins remain poorly characterized. Here, we show that MpACR3, the ACR3 ortholog from the model liverwort Marchantia polymorpha, contains an arsenic-responsive N-terminal domain that regulates intracellular trafficking. Under non-stress conditions, MpACR3 is detected in Golgi bodies as well as at the plasma membrane in plant cells, whereas As(III) promotes its relocalization to the plasma membrane. As(III) interacts with a cysteine-rich N-terminal region and stabilizes this domain, as shown by arsenite-biotin competition and increased protease resistance. Together with molecular dynamics simulations, which predicts As(III)-dependent changes in secondary structure and surface properties, these findings support a model in which ligand-induced remodeling of the N terminus modulates MpACR3 retention and trafficking. We further identify a di-arginine-containing LRCRF motif in which leucine and arginine residues are required for intracellular retention, whereas cysteine and phenylalanine residues are required for As(III)-induced relocalization to the plasma membrane. The broad conservation of related N-terminal domains in plant ACR3 transporters suggests that arsenic-regulated trafficking is a conserved plant adaptation to arsenic toxicity.