Calcium-Promoted Interaction between the C2-Domain Protein EHB1 and Metal Transporter IRT1 Inhibits Arabidopsis Iron Acquisition

Abstract

Iron is a key transition element in the biosphere and is crucial for living organisms, although its cellular excess can be deleterious. Maintaining the balance of optimal iron availability in the model plant Arabidopsis (Arabidopsis thaliana) requires the precise operation of iron import through the principal iron transporter IRON-REGULATED TRANSPORTER1 (IRT1). Targeted inhibition of IRT1 can prevent oxidative stress, thus promoting plant survival. Here, we report the identification of an IRT1 inhibitor, namely the C2 domain-containing peripheral membrane protein ENHANCED BENDING1 (EHB1). EHB1 interacts with the cytoplasmically exposed variable region of IRT1, and we demonstrate that this interaction is greatly promoted by the presence of calcium. We found that EHB1 binds lipids characteristic of the plasma membrane, and the interaction between EHB1 and plant membranes is calcium-dependent. Molecular simulations showed that EHB1 membrane binding is a two-step process that precedes the interaction between EHB1 and IRT1. Genetic and physiological analyses indicated that EHB1 acts as a negative regulator of iron acquisition. The presence of EHB1 prevented the IRT1-mediated complementation of iron-deficient fet3fet4 yeast (Saccharomyces cerevisiae). Our data suggest that EHB1 acts as a direct inhibitor of IRT1-mediated iron import into the cell. These findings represent a major step in understanding plant iron acquisition, a process that underlies the primary production of bioavailable iron for land ecosystems.