Altered selectivity in an Arabidopsis metal transporter

Plants require metals for essential functions ranging from respiration to photosynthesis. These metals also contribute to the nutritional value of plants for both humans and livestock. Additionally, plants have the ability to accumulate nonessential metals such as cadmium and lead, and this ability could be harnessed to remove pollutant metals from the environment. Designing a transporter that specifically accumulates certain cations while excluding others has exciting applications in all of these areas. The Arabidopsis root membrane protein IRT1 is likely to be responsible for uptake of iron from the soil. Like other Fe(II) transporters identified to date, IRT1 transports a variety of other cations, including the essential metals zinc and manganese as well as the toxic metal cadmium. By heterologous expression in yeast, we show here that the replacement of a glutamic acid residue at position 103 in wild-type IRT1 with alanine increases the substrate specificity of the transporter by selectively eliminating its ability to transport zinc. Two other mutations, replacing the aspartic acid residues at either positions 100 or 136 with alanine, also increase IRT1 metal selectivity by eliminating transport of both iron and manganese. A number of other conserved residues in or near transmembrane domains appear to be essential for all transport function. Therefore, this study identifies at least some of the residues important for substrate selection and transport in a protein belonging to the ZIP gene family, a large transporter family found in a wide variety of organisms.