Altered selectivity in an Arabidopsis metal transporter

Plants require metals for essential functions ranging from respiration to p hotosynthesis. These metals also contribute to the nutritional value of pla nts 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. Designi ng a transporter that specifically accumulates certain cations While exclud ing 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(ll) transporters identified to date, IRT1 tran sports 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 pos ition 103 in wild-type IRT1 with alanine increases the substrate specificit y of the transporter by selectively eliminating its ability to transport zi nc. Two other mutations, replacing the aspartic acid residues at either pos itions 100 or 136 with alanine, also increase IRT1 metal selectivity by eli minating transport of both iron and manganese. A number of other conserved residues in or near transmembrane domains appear to be essential for all tr ansport function. Therefore, this study identifies at least some of the res idues important for substrate selection and transport in a protein belongin g to the ZIP gene family, a large transporter family found in a wide variet y of organisms.