La Fontaine Sharon, Quinn Jeanette M, Nakamoto Stacie S, Page M Dudley, Göhre Vera, Moseley Jeffrey L, Kropat Janette, Merchant Sabeeha
Department of Chemistry and Biochemistry, University of California, Los Angeles 90095-1569, USA.
Eukaryot Cell. 2002 Oct;1(5):736-57. doi: 10.1128/EC.1.5.736-757.2002.
The unicellular green alga Chlamydomonas reinhardtii is a valuable model for studying metal metabolism in a photosynthetic background. A search of the Chlamydomonas expressed sequence tag database led to the identification of several components that form a copper-dependent iron assimilation pathway related to the high-affinity iron uptake pathway defined originally for Saccharomyces cerevisiae. They include a multicopper ferroxidase (encoded by Fox1), an iron permease (encoded by Ftr1), a copper chaperone (encoded byAtx1), and a copper-transporting ATPase. A cDNA, Fer1, encoding ferritin for iron storage also was identified. Expression analysis demonstrated that Fox1 and Ftrl were coordinately induced by iron deficiency, as were Atx1 and Fer1, although to lesser extents. In addition, Fox1 abundance was regulated at the posttranscriptional level by copper availability. Each component exhibited sequence relationship with its yeast, mammalian, or plant counterparts to various degrees; Atx1 of C. reinhardtii is also functionally related with respect to copper chaperone and antioxidant activities. Fox1 is most highly related to the mammalian homologues hephaestin and ceruloplasmin; its occurrence and pattern of expression in Chlamydomonas indicate, for the first time, a role for copper in iron assimilation in a photosynthetic species. Nevertheless, growth of C. reinhardtii under copper- and iron-limiting conditions showed that, unlike the situation in yeast and mammals, where copper deficiency results in a secondary iron deficiency, copper-deficient Chlamydomonas cells do not exhibit symptoms of iron deficiency. We propose the existence of a copper-independent iron assimilation pathway in this organism.
单细胞绿藻莱茵衣藻是研究光合背景下金属代谢的宝贵模型。对莱茵衣藻表达序列标签数据库的搜索，导致鉴定出几个组成部分，它们形成了一条与最初为酿酒酵母定义的高亲和力铁摄取途径相关的铜依赖性铁同化途径。它们包括一种多铜铁氧化酶（由Fox1编码）、一种铁通透酶（由Ftr1编码）、一种铜伴侣蛋白（由Atx1编码）和一种铜转运ATP酶。还鉴定出一个编码用于铁储存的铁蛋白的cDNA Fer1。表达分析表明，Fox1和Ftrl在缺铁时被协同诱导，Atx1和Fer1也是如此，尽管程度较小。此外，Fox1的丰度在转录后水平受铜可用性的调节。每个组分与其酵母、哺乳动物或植物对应物在不同程度上表现出序列关系；莱茵衣藻的Atx1在铜伴侣和抗氧化活性方面也具有功能相关性。Fox1与哺乳动物同源物血蓝蛋白和铜蓝蛋白的关系最为密切；它在莱茵衣藻中的出现和表达模式首次表明铜在光合物种的铁同化中起作用。然而，莱茵衣藻在铜和铁限制条件下的生长表明，与酵母和哺乳动物不同，在酵母和哺乳动物中铜缺乏会导致继发性铁缺乏，而铜缺乏的莱茵衣藻细胞没有表现出铁缺乏的症状。我们提出在这种生物体中存在一条不依赖铜的铁同化途径。
