Department of Molecular Virology and Microbiology, Baylor College of Medicine, Houston, Texas 77030, USA.
J Biol Chem. 2011 Sep 23;286(38):33652-60. doi: 10.1074/jbc.M111.241687. Epub 2011 Aug 1.
Pathogenic bacteria require iron to replicate inside mammalian hosts. Recent studies indicate that heme acquisition in Gram-positive bacteria is mediated by proteins containing one or more near-iron transporter (NEAT) domains. Bacillus anthracis is a spore-forming, Gram-positive pathogen and the causative agent of anthrax disease. The rapid, extensive, and efficient replication of B. anthracis in host tissues makes this pathogen an excellent model organism for the study of bacterial heme acquisition. B. anthracis secretes two NEAT hemophores, IsdX1 and IsdX2. IsdX1 contains a single NEAT domain, whereas IsdX2 has five, a novel property among hemophores. To understand the functional significance of harboring multiple, non-identical NEAT domains, we purified each individual NEAT domain of IsdX2 as a GST fusion and analyzed the specific function of each domain as it relates to heme acquisition and transport. NEAT domains 1, 3, 4, and 5 all bind heme, with domain 5 having the highest affinity. All NEATs associate with hemoglobin, but only NEAT1 and -5 can extract heme from hemoglobin, seemingly by a specific and active process. NEAT1, -3, and -4 transfer heme to IsdC, a cell wall-anchored anthrax NEAT protein. These results indicate that IsdX2 has all the features required to acquire heme from the host and transport heme to the bacterial cell wall. Additionally, these results suggest that IsdX2 may accelerate iron import rates by acting as a "heme sponge" that enhances B. anthracis replication in iron-starved environments.
致病细菌需要铁才能在哺乳动物宿主内复制。最近的研究表明，革兰氏阳性菌中的血红素获取是由含有一个或多个近铁转运蛋白 (NEAT) 结构域的蛋白质介导的。炭疽芽孢杆菌是一种形成孢子的革兰氏阳性病原体，也是炭疽病的病原体。炭疽芽孢杆菌在宿主组织中的快速、广泛和高效复制，使其成为研究细菌血红素获取的理想模型生物。炭疽芽孢杆菌分泌两种 NEAT 血红素载体，IsdX1 和 IsdX2。IsdX1 含有一个 NEAT 结构域，而 IsdX2 有五个，这在血红素载体中是一个新颖的特性。为了了解拥有多个非同源 NEAT 结构域的功能意义，我们将 IsdX2 的每个单独的 NEAT 结构域作为 GST 融合蛋白进行纯化，并分析了每个结构域在血红素获取和运输方面的特定功能。NEAT 结构域 1、3、4 和 5 都与血红素结合，其中结构域 5 的亲和力最高。所有 NEAT 都与血红蛋白结合，但只有 NEAT1 和 -5 可以从血红蛋白中提取血红素，似乎是通过特定和活跃的过程。NEAT1、-3 和 -4 将血红素转移到 IsdC，一种细胞壁锚定的炭疽 NEAT 蛋白。这些结果表明，IsdX2 具有从宿主获取血红素并将血红素转运到细菌细胞壁所需的所有特征。此外，这些结果表明，IsdX2 可能通过充当“血红素海绵”来加速铁的导入速率，从而增强缺铁环境中炭疽芽孢杆菌的复制。
