Unité des Membranes Bactériennes, CNRS URA 2172, Département de Microbiologie, Institut Pasteur, 25 rue du Dr Roux, 75724 Paris Cedex 15, France.
Mol Microbiol. 2011 Apr;80(1):133-48. doi: 10.1111/j.1365-2958.2011.07562.x. Epub 2011 Feb 15.
Haemophilus influenzae is an obligate human commensal/pathogen. This haem auxotroph must acquire haem from its host to sustain aerobic growth. Haem-haemopexin complexes are one of the potential sources of haem for this microorganism. Haemopexin is a glycoprotein that binds haem with high affinity (subpicomolar Kd) and involved in haem recycling. HxuA, a cell surface protein, is the key to haem acquisition from haemopexin. In this study, we reconstituted a functional Hxu system from H. influenzae in Escherichia coli K-12 that mediated active haem transport across the outer membrane from haem-haemopexin, in the presence of the inner membrane energy-transducing TonB-ExbB-ExbD complex from H. influenzae. A secreted variant of HxuA, HxuA(dm), was produced in E. coli. HxuA(dm) functionally complemented an hxuA mutant of H. influenzae for haem-haemopexin acquisition. HxuA(dm) interacted with haemopexin and haem-haemopexin, with which it formed high-affinity, stoichiometric complexes. Following the interaction between haem-haemopexin and HxuA(dm), haem was no longer bound to its initial high-affinity site and became accessible to its cognate haem receptor, HxuC. HxuA(dm) and the HxuA(dm)-haemopexin complex do not appear to bind haem at detectable levels (affinities below 10(6) M(-1)). HxuA thus appears to 'release' haem from haem-haemopexin complexes and to prevent haem sequestering by haemopexin.
流感嗜血杆菌是一种专性人共生/病原体。这种血红素营养缺陷型必须从宿主中获取血红素以维持需氧生长。血红素-结合珠蛋白复合物是该微生物血红素的潜在来源之一。结合珠蛋白是一种糖蛋白，与血红素有高亲和力（亚皮摩尔 Kd）结合，并参与血红素循环利用。细胞表面蛋白 HxuA 是从结合珠蛋白中获取血红素的关键。在这项研究中，我们在大肠杆菌 K-12 中重建了具有功能的 Hxu 系统，该系统介导了来自血红素-结合珠蛋白的活性血红素跨外膜转运，同时存在来自流感嗜血杆菌的内膜能量转导 TonB-ExbB-ExbD 复合物。HxuA 的分泌变体 HxuA(dm)在大肠杆菌中产生。HxuA(dm)在功能上补充了流感嗜血杆菌 hxuA 突变体的血红素-结合珠蛋白获取。HxuA(dm)与结合珠蛋白和血红素-结合珠蛋白相互作用，形成高亲和力、化学计量比的复合物。在血红素-结合珠蛋白与 HxuA(dm)相互作用后，血红素不再与最初的高亲和力位点结合，并且可以与同源血红素受体 HxuC 结合。HxuA(dm)和 HxuA(dm)-结合珠蛋白复合物似乎不以可检测水平（亲和力低于 10(6) M(-1)）结合血红素。因此，HxuA 似乎可以从血红素-结合珠蛋白复合物中“释放”血红素，并防止结合珠蛋白对血红素的隔离。
