Cryo-EM provides insight into how the Staphylococcus aureus IsdH receptor removes hemin from the hemoglobin:haptoglobin complex
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Staphylococcus aureus extracts hemin from human hemoglobin (Hb) to overcome host-imposed iron limitation. How it recovers Hb-bound hemin from the hemoglobin:haptoglobin (Hb:Hp) complex, the major circulating form of Hb outside red blood cells, remains unclear. Here we use cryo-electron microscopy, biophysical measurements, and solution kinetics to define how the S. aureus IsdH surface receptor extracts hemin from Hb:Hp. A 3.1 Å cryo-EM structure of Hb:Hp bound by full-length IsdH reveals that its N-terminal NEAT domain (N1) anchors it to αHb, whereas its downstream N2N3 extraction unit engages βHb to remove its hemin. The receptor engages Hb:Hp differently than isolated Hb, because N-linked glycans on haptoglobin bias the extraction unit toward βHb, sterically occluding its access to αHb while still permitting engagement by N1. Kinetic assays show that IsdH actively accelerates hemin release from Hb:Hp. Three-dimensional variability analysis indicates that this likely occurs via a dynamic interface in which receptor motions reposition the extraction unit relative to βHb, collectively supporting a model in which IsdH transiently perturbs the F-helix to promote hemin extraction. Alignment of that model with a previously determined CD163:Hb:Hp structure shows how IsdH may disrupt Hb:Hp recognition by macrophage and monocyte CD163 receptors, helping to explain how it may hinder clearance of Hb:Hp from circulation. In aggregate, these results help define the structural basis for hemin extraction from Hb:Hp and how IsdH may subvert receptor-mediated clearance of the Hb:Hp complex.
Significance
Staphylococcus aureus scavenges hemin from host hemoglobin to proliferate, yet most extracellular hemoglobin is sequestered in hemoglobin:haptoglobin (Hb:Hp) complexes, which are rapidly cleared from circulation. To clarify how bacteria access hemin in this context, we now show that the IsdH surface receptor is structurally adapted to extract hemin from Hb:Hp. Our results indicate that IsdH uses distinct NEAT domains to anchor to αHb and selectively extract hemin from βHb. This βHb selectivity is shaped by haptoglobin N-glycans and enhances microbial access to iron. These findings provide a mechanistic framework for targeting heme acquisition as an anti-virulence strategy.
