Cell Biology of Iron Transport

我们已经使用全基因组方法来确定在萌芽酵母中参与铁缺乏和铁超载反应的基因。酵母对缺铁的反应是通过激活铁摄取系统，动员储存的铁，并转移到铁不依赖的代谢途径。在缺铁期间激活的一种铁摄取系统专门运输铁-铁载体螯合物，如铁铬(Fc)。酿酒酵母中Fc转运蛋白Arn1的胞内转运部分受Fc与转运蛋白结合的控制。在没有Fc的情况下，Arn1直接从高尔基体到内体。Fc结合触发Arn1重新分布到质膜，而Fc转运与Arn1在质膜和内体之间的循环有关。我们确定Arn1的运输需要网状蛋白适配器GGa2和Rsp5泛素连接酶的泛素化。高尔基体需要GGa2来运输Arn1，Arn1是从内体分离到液泡中降解的。转运到液泡腔依赖于RSP5的泛素化，但在铁酶存在下，Arn1的质膜积累不需要泛素化。通过逆转录复合体或Snx4逆行运输也不是质膜积累所必需的。高浓度的FC导致Arn1泛素化水平较高，但不会导致降解。如果没有这种泛素化，Arn1仍然留在质膜上，在那里它是活跃的运输。Arn1在转运蛋白氨基末端的一簇赖氨酸残基上优先被多泛素链修饰。进一步的研究表明，GGa2也参与了Arn1在质膜上的内吞作用，但GGa2并不需要泛素泛素残基的结合来介导Arn1的转运。

We have used a whole-genome approach to identify genes that are involved in the response to iron deprivation and iron overload in the budding yeast, Saccharomyces cerevisiae. Yeast respond to iron deprivation by activating systems of iron uptake, by mobilizing stored iron, and by shifting to iron-independent metabolic pathways. One type of iron uptake system activated during iron deprivation specifically transports iron-siderophore chelates, such as ferrichrome (FC). The intracellular trafficking of Arn1, a FC transporter in Saccharomyces cerevisiae, is controlled in part by the binding of FC to the transporter. In the absence of FC, Arn1 is sorted directly from the Golgi to endosomes. FC binding triggers the redistribution of Arn1 to the plasma membrane, while FC transport is associated with the cycling of Arn1 between the plasma membrane and endosomes. We determined that the clathrin adaptor Gga2 and ubiquitination by the Rsp5 ubiquitin ligase are required for trafficking of Arn1. Gga2 was required for Golgi to endosomal trafficking of Arn1, which was sorted from endosomes to the vacuole for degradation. Trafficking into the vacuolar lumen was dependent on ubiquitination by Rsp5, but ubiquitination was not required for plasma membrane accumulation of Arn1 in the presence of ferrichrome. Retrograde trafficking via the retromer complex or Snx4 was also not required for plasma membrane accumulation. High concentrations of FC led to higher levels of ubiquitination of Arn1, but did not induce degradation. Without this ubiquitination, Arn1 remained on the plasma membrane, where it was active for transport. Arn1 was preferentially modified with poly-ubiquitin chains on a cluster of lysine residues at the amino terminus of the transporter. Further studies have revealed that Gga2 is also involved in the endocytosis of Arn1 at the plasma membrane, but that the binding of ubiquitin ubiquitin residues is not required for Gga2 to mediate trafficking of Arn1.