Schaaf Gabriel, Ludewig Uwe, Erenoglu Bülent E, Mori Satoshi, Kitahara Takeshi, von Wirén Nicolaus
Institut für Pflanzenernährung, Universität Hohenheim, D-70593 Stuttgart, Germany.
J Biol Chem. 2004 Mar 5;279(10):9091-6. doi: 10.1074/jbc.M311799200. Epub 2003 Dec 29.
Among higher plants graminaceous species have the unique ability to efficiently acquire iron from alkaline soils with low iron solubility by secreting phytosiderophores, which are hexadentate metal chelators with high affinity for Fe(III). Iron(III)-phytosiderophores are subsequently taken up by roots via YS1 transporters, that belong to the OPT oligopeptide transporter family. Despite its physiological importance at alkaline pH, uptake of Fe-phytosiderophores into roots of wild-type maize plants was greater at acidic pH and sensitive to the proton uncoupler CCCP. To access the mechanism of Fe-phytosiderophore acquisition, ZmYS1 was expressed in an iron uptake-defective yeast mutant and in Xenopus oocytes, where ZmYS1-dependent Fe-phytosiderophore transport was stimulated at acidic pH and sensitive to CCCP. Electrophysiological analysis in oocytes demonstrated that Fephytosiderophore transport depends on proton cotransport and on the membrane potential, which allows ZmYS1-mediated transport even at alkaline pH. We further investigated substrate specificity and observed that ZmYS1 complemented the growth defect of the zinc uptake-defective yeast mutant zap1 and transported various phytosiderophore-bound metals into oocytes, including zinc, copper, nickel, and, at a lower rate, also manganese and cadmium. Unexpectedly, ZmYS1 also transported Ni(II), Fe(II), and Fe(III) complexes with nicotianamine, a structural analog of phytosiderophores, which has been shown to act as an intracellular metal chelator in all higher plants. Our results show that ZmYS1 encodes a proton-coupled broad-range metal-phytosiderophore transporter that additionally transports Fe- and Ni-nicotianamine. These biochemical properties indicate a novel role of YS1 transporters for heavy metal homeostasis in plants.
在高等植物中，禾本科物种具有独特的能力，能够通过分泌植物铁载体从铁溶解度低的碱性土壤中高效获取铁。植物铁载体是对Fe(III)具有高亲和力的六齿金属螯合剂。随后，铁(III)-植物铁载体通过属于OPT寡肽转运蛋白家族的YS1转运蛋白被根吸收。尽管在碱性pH条件下其具有生理重要性，但野生型玉米植株根对铁-植物铁载体的吸收在酸性pH条件下更大，且对质子解偶联剂CCCP敏感。为了探究铁-植物铁载体的获取机制，ZmYS1在铁吸收缺陷型酵母突变体和非洲爪蟾卵母细胞中表达，在这些细胞中，ZmYS1依赖的铁-植物铁载体转运在酸性pH条件下受到刺激，且对CCCP敏感。卵母细胞的电生理分析表明，铁-植物铁载体转运依赖于质子共转运和膜电位，这使得ZmYS1介导的转运即使在碱性pH条件下也能进行。我们进一步研究了底物特异性，发现ZmYS1弥补了锌吸收缺陷型酵母突变体zap1的生长缺陷，并将各种植物铁载体结合的金属转运到卵母细胞中，包括锌、铜、镍，以及较低速率的锰和镉。出乎意料的是，ZmYS1还转运了Ni(II)、Fe(II)和Fe(III)与烟酰胺的复合物，烟酰胺是植物铁载体的结构类似物，已被证明在所有高等植物中作为细胞内金属螯合剂发挥作用。我们的结果表明，ZmYS1编码一种质子偶联的广谱金属-植物铁载体转运蛋白，此外还转运铁-烟酰胺和镍-烟酰胺。这些生化特性表明YS1转运蛋白在植物重金属稳态中具有新的作用。
