Protchenko Olga, Rodriguez-Suarez Roberto, Androphy Rachel, Bussey Howard, Philpott Caroline C
Liver Diseases Branch, NIDDK, National Institutes of Health, Bethesda, Maryland 20892.
Department of Biology, McGill University, Montreal, Quebec H3A 1B1, Canada.
J Biol Chem. 2006 Jul 28;281(30):21445-21457. doi: 10.1074/jbc.M512812200. Epub 2006 May 22.
Although Candida albicans and Saccharomyces cerevisiae express very similar systems of iron uptake, these species differ in their capacity to use heme as a nutritional iron source. Whereas C. albicans efficiently takes up heme, S. cerevisiae grows poorly on media containing heme as the sole source of iron. We identified a gene from C. albicans that would enhance heme uptake when expressed in S. cerevisiae. Overexpression of CaFLC1 (for flavin carrier 1) stimulated the growth of S. cerevisiae on media containing heme iron. In C. albicans, deletion of both alleles of CaFLC1 resulted in a decrease in heme uptake activity, whereas overexpression of CaFLC1 resulted in an increase in heme uptake. The S. cerevisiae genome contains three genes with homology to CaFLC1, and two of these, termed FLC1 and FLC2, also stimulated growth on heme when overexpressed in S. cerevisiae. The S. cerevisiae Flc proteins were detected in the endoplasmic reticulum and the FLC genes encoded an essential function, as strains deleted for either FLC1 or FLC2 were viable, but deletion of both FLC1 and FLC2 was synthetically lethal. FLC gene deletion resulted in pleiotropic phenotypes related to defects in cell wall integrity. High copy suppressors of this synthetic lethality included three mannosyltransferases, VAN1, KTR4, and HOC1. FLC deletion strains exhibited loss of cell wall mannose phosphates, defects in cell wall assembly, and delayed maturation of carboxypeptidase Y. Permeabilized cells lacking FLC proteins exhibited dramatic loss of FAD import activity. We propose that the FLC genes are required for import of FAD into the lumen of the endoplasmic reticulum, where it is required for disulfide bond formation.
尽管白色念珠菌和酿酒酵母表达非常相似的铁摄取系统，但这些物种在利用血红素作为营养性铁源的能力上存在差异。白色念珠菌能够有效地摄取血红素，而酿酒酵母在以血红素作为唯一铁源的培养基上生长较差。我们从白色念珠菌中鉴定出一个基因，当它在酿酒酵母中表达时会增强血红素摄取。CaFLC1（黄素载体1）的过表达刺激了酿酒酵母在含血红素铁的培养基上的生长。在白色念珠菌中，CaFLC1两个等位基因的缺失导致血红素摄取活性降低，而CaFLC1的过表达导致血红素摄取增加。酿酒酵母基因组包含三个与CaFLC1同源的基因，其中两个，称为FLC1和FLC2，在酿酒酵母中过表达时也能刺激在血红素上的生长。酿酒酵母的Flc蛋白在内质网中被检测到，并且FLC基因编码一种必需功能，因为缺失FLC1或FLC2的菌株是存活的，但同时缺失FLC1和FLC2是合成致死的。FLC基因缺失导致与细胞壁完整性缺陷相关的多效性表型。这种合成致死性的高拷贝抑制子包括三种甘露糖基转移酶，VAN1、KTR4和HOC1。FLC缺失菌株表现出细胞壁甘露糖磷酸的丢失、细胞壁组装缺陷以及羧肽酶Y成熟延迟。缺乏Flc蛋白的透化细胞表现出FAD导入活性的显著丧失。我们提出FLC基因是FAD导入内质网腔所必需的，而在内质网腔中FAD是二硫键形成所必需的。
