Eukaryotic Heme Utilization

人们对真核生物中的血红素转运知之甚少，酵母中也没有鉴定出一种特定的血红素转运系统。尽管酿酒酵母不会对缺铁作出反应，但我们已经确定该物种确实有一个可诱导的血红素吸收系统，该系统在缺铁和低氧条件下被激活。我们对在这些条件下生长的菌株进行了微阵列分析，发现一些假定的转运蛋白在转录上被激活。 血红素是在线粒体中合成的，但辅因子在整个细胞的隔间中使用。血红素是高度疏水的，不太可能被释放到胞浆的水环境中，这表明存在着血红素在细胞内运输的机制。我们已经设计了一种基因筛查，用于鉴定参与细胞内血红素运输的酵母蛋白，并正在评估候选蛋白。在这个屏幕上发现的基因表明，血红素通过囊泡运输系统从线粒体移动到其他位置。 与马里兰大学的Iqbal Hamza合作，我们正在通过在酵母中表达线虫可能的血红素转运蛋白来对它们进行功能表征。线虫的血红素转运体在酵母中表达时具有吸收血红素的活性，而突变形式的血红素转运体在酵母中表达时会失去功能。

Very little is known about heme transport in eukaryotes and a heme-specific transport uptake systems have not been identified in yeast. Although S. cerevisiae does not take up heme in response to iron deprivation, we have determined that this species does have an inducible heme uptake system that is activated under heme deficiency and hypoxic conditions. We performed microarray analysis of strains grown under these conditions, and found that a number of putative transporters are transcriptionally activated. Heme is synthesized in mitochondria, yet the cofactor is used in compartments throughout the cell. Heme is highly hydrophobic and is unlikely to be released into the aqueous environment of the cytosol, suggesting that mechanisms exist for the intracellular trafficking of heme. We have designed a genetic screen for the identification of yeast proteins involved in intracellular heme trafficking and are evaluating candidates. Genes identified in this screen suggest that heme moves from the mitochondria to other sites through the vesicular trafficking system. In collaboration with Iqbal Hamza of the University of Maryland, we are functionally characterizing the putative heme transporters of C. elegans by expressing them in yeast. Heme transporters from C. elegans have heme uptake activity when expressed in yeast and mutant forms of heme transporters exhibit loss of function when expressed in yeast.