A Relay Network of Extracellular Heme-Binding Proteins Drives C. albicans Iron Acquisition from Hemoglobin

Iron scavenging constitutes a crucial challenge for survival of pathogenic microorganisms in the iron-poor host environment. Candida albicans, like many microbial pathogens, is able to utilize iron from hemoglobin, the largest iron pool in the host's body. Rbt5 is an extracellular glycosylphosphatidylinositol (GPI)-anchored heme-binding protein of the CFEM family that facilitates heme-iron uptake by an unknown mechanism. Here, we characterize an additional C. albicans CFEM protein gene, PGA7, deletion of which elicits a more severe heme-iron utilization phenotype than deletion of RBT5. The virulence of the pga7−/− mutant is reduced in a mouse model of systemic infection, consistent with a requirement for heme-iron utilization for C. albicans pathogenicity. The Pga7 and Rbt5 proteins exhibit distinct cell wall attachment, and discrete localization within the cell envelope, with Rbt5 being more exposed than Pga7. Both proteins are shown here to efficiently extract heme from hemoglobin. Surprisingly, while Pga7 has a higher affinity for heme in vitro, we find that heme transfer can occur bi-directionally between Pga7 and Rbt5, supporting a model in which they cooperate in a heme-acquisition relay. Together, our data delineate the roles of Pga7 and Rbt5 in a cell surface protein network that transfers heme from extracellular hemoglobin to the endocytic pathway, and provide a paradigm for how receptors embedded in the cell wall matrix can mediate nutrient uptake across the fungal cell envelope.在缺铁的宿主环境中，铁获取（Iron scavenging）是病原微生物存活所面临的关键挑战。与诸多病原微生物一样，白色念珠菌（Candida albicans）能够利用宿主体内最大的铁储备——血红蛋白中的铁。Rbt5是一种胞外糖基磷脂酰肌醇（GPI）锚定的CFEM家族血红素结合蛋白，可通过未知机制促进血红素铁的摄取。本研究中，我们对另一种白色念珠菌CFEM家族蛋白基因PGA7进行了功能表征；与敲除RBT5相比，敲除PGA7会引发更为严重的血红素铁利用缺陷表型。在全身性感染小鼠模型中，pga7−/−突变体的毒力显著降低，这与白色念珠菌的致病性依赖血红素铁利用的结论相符。Pga7与Rbt5蛋白呈现出不同的细胞壁附着模式，且在细胞被膜内的定位存在显著差异：Rbt5的暴露程度高于Pga7。本研究证实，两种蛋白均可高效从血红蛋白中提取血红素。令人意外的是，尽管Pga7在体外对血红素的亲和力更高，但我们发现血红素可在Pga7与Rbt5之间双向传递，这支持二者通过血红素获取接力协作的工作模型。综上，我们的研究数据明确了Pga7与Rbt5在细胞表面蛋白网络中的功能角色：该网络可将胞外血红蛋白中的血红素转运至内吞途径，同时也为嵌入细胞壁基质的受体如何介导营养物质跨真菌细胞被膜摄取提供了研究范式。