Identification of an Outer Membrane Region Required for the Substrate Specificity of Fe(III)-phytosiderophore Transporter HvYS1 in Barley

Abstract

We have identified a gene encoding an iron-phytosiderophore transporter (HvYS1) in barley. HvYS1 is a specific transporter for Fe(III)-phytosiderophores, and involved in primary iron acquisition from soils in barley roots. In contrast, ZmYS1 in maize possesses broad substrate specificity, despite a high homology with HvYS1. The distinct difference in the substrate specificities between ZmYS1 and HvYS1 motivated us to investigate the mechanism by which the transporters distinguish their substrates. In this study we revealed, by assessing the transport activity of a series of HvYS1-ZmYS1 chimeras, that the outer membrane loop between the 6th and 7th transmembrane regions is essential for the substrate specificity. Circular dichroism spectra indicated that a synthetic peptide corresponding to the loop of HvYS1 forms an α-helix in solution, whereas that of ZmYS1 is flexible. We propose that the structural difference at this particular loop determines the substrate specificity of the HvYS1 transporter.