Structural-Functional Analysis of Membrane Transporters Underlying Plant Aluminum Tolerance

Many plants species deal with aluminum (Al) toxicity that limits crop production on acid soils by releasing organic acid anions from the root, thereby immobilizing Al³⁺ in the rhizosphere and preventing it from entering the root. Members of the ALMT (Al-activated malate transporter) and MATE (multidrug and toxic compound efflux) transporter families mediate these transport processes. We have integrated electrophysiological analysis and cellular imaging approaches to conduct a structural-functional analysis of ALMTs, allowing us to determine the protein topology and dissect the role of specific protein domains in the functional properties of these transporters. Our results indicate that the intracellular N-domain forms a conductive pathway that mediates ion transport even in the absence of the intracellular C-domain. However, regions in the N-terminus and a hydrophobic region at the C-terminus appear to jointly be involved in Al³⁺-sensing leading to the well documented Al enhancement of transport (malate efflux) activity. A characteristic motif in the N-terminus appears to be specific to the Al-responsive ALMTs. The Al enhancement of organic acid efflux is likely an important intrinsic regulatory mechanism that helps to limit unnecessary root carbon loss by ensuring that maximal malate release occurs in the presence of toxic Al in the rhizosphere. These studies are allowing us to identify protein residues/ motifs underlying transport properties critical for Al tolerance, with the ultimate goal of "engineering" these transporters to enhance their ability to confer Al tolerance in crop plants grown on acid soils.