Paz Yakov, Katz Adriana, Pick Uri
Department of Biological Chemistry, The Weizmann Institute of Science, Rehovot 76100, Israel.
J Biol Chem. 2007 Mar 23;282(12):8658-66. doi: 10.1074/jbc.M609756200. Epub 2007 Jan 16.
The halotolerant alga Dunaliella salina is unique among plants in that it utilizes a transferrin (TTf) to mediate iron acquisition (Fisher, M., Zamir, A., and Pick, U. (1998) J. Biol. Chem. 273, 17553-17558). Two new proteins that are induced by iron deprivation were identified in plasma membranes of D. salina as follows: a multicopper ferroxidase termed D-Fox and an internally duplicated glycoprotein (p130B). D-Fox and p130B are accessible to glycolytic, proteolytic, and biotin surface tagging treatments, suggesting that they are surface-exposed glycoproteins. Induction of D-Fox was also manifested by ferroxidase activity in plasma membrane preparations. These results are puzzling because ferroxidases in yeast and in Chlamydomonas reinhardtii function in redox-mediated iron uptake, a mechanism that is not known to operate in D. salina. Two lines of evidence suggest that D-Fox and p130B interact with D. salina triplicated transferrin (TTf). First, chemical cross-linking combined with mass spectroscopy analysis showed that D-Fox and p130B associate with TTf and with another plasma membrane transferrin. Second, detergent-solubilized D-Fox and p130B comigrated on blue native gels with plasma membrane transferrins. 59Fe autoradiography indicated that this complex binds Fe3+ ions. Also, the induction of D-Fox and p130B is kinetically correlated with enhanced iron binding and uptake activities. These results suggest that D-Fox and p130B associate with plasma membrane transferrins forming a complex that enhances iron binding and iron uptake. We propose that the function of D-Fox in D. salina has been modified during evolution from redox-mediated to transferrin-mediated iron uptake, following a gene transfer event of transferrins from an ancestral animal cell.
嗜盐藻类杜氏盐藻在植物中独具特色，它利用一种转铁蛋白（TTf）来介导铁的摄取（Fisher, M., Zamir, A., and Pick, U. (1998) J. Biol. Chem. 273, 17553 - 17558）。在杜氏盐藻的质膜中鉴定出两种因缺铁而被诱导产生的新蛋白质：一种称为D - Fox的多铜铁氧化酶和一种内部重复的糖蛋白（p130B）。D - Fox和p130B可被糖酵解、蛋白水解和生物素表面标记处理所作用，这表明它们是表面暴露的糖蛋白。质膜制剂中的铁氧化酶活性也体现了D - Fox的诱导情况。这些结果令人困惑，因为酵母和莱茵衣藻中的铁氧化酶在氧化还原介导的铁摄取中起作用，而这种机制在杜氏盐藻中并不为人所知。有两条证据表明D - Fox和p130B与杜氏盐藻的三聚体转铁蛋白（TTf）相互作用。首先，化学交联结合质谱分析表明D - Fox和p130B与TTf以及另一种质膜转铁蛋白相关联。其次，去污剂溶解的D - Fox和p130B在蓝色非变性凝胶上与质膜转铁蛋白一起迁移。59Fe放射自显影表明这种复合物结合Fe3 +离子。此外，D - Fox和p130B的诱导在动力学上与增强的铁结合和摄取活性相关。这些结果表明D - Fox和p130B与质膜转铁蛋白结合形成一种复合物，该复合物增强了铁的结合和摄取。我们推测，在转铁蛋白从祖先动物细胞发生基因转移事件后，杜氏盐藻中D - Fox的功能在从氧化还原介导的铁摄取进化到转铁蛋白介导的铁摄取的过程中发生了改变。
