NEAr Transporter (NEAT) Domains: Unique Surface Displayed Heme Chaperones That Enable Gram-Positive Bacteria to Capture Heme-Iron From Hemoglobin

Abstract

Iron is an important micronutrient that is required by bacteria to proliferate and to cause disease. Many bacterial pathogens forage iron from human hemoglobin (Hb) during infections, which contains this metal within heme (iron–protoporphyrin IX). Several clinically important pathogenic species within the Firmicutes phylum scavenge heme using surface-displayed or secreted NEAr Transporter (NEAT) domains. In this review, we discuss how these versatile proteins function in the Staphylococcus aureus Iron-regulated surface determinant system that scavenges heme-iron from Hb. S. aureus NEAT domains function as either Hb receptors or as heme-binding chaperones. In vitro studies have shown that heme-binding NEAT domains can rapidly exchange heme amongst one another via transiently forming transfer complexes, leading to the interesting hypothesis that they may form a protein-wire within the peptidoglycan layer through which heme flows from the microbial surface to the membrane. In Hb receptors, recent studies have revealed how dedicated heme- and Hb-binding NEAT domains function synergistically to extract Hb’s heme molecules, and how receptor binding to the Hb-haptoglobin complex may block its clearance by macrophages, prolonging microbial access to Hb’s iron. The functions of NEAT domains in other Gram-positive bacteria are also reviewed.

Cryo-EM structures of staphylococcal IsdB bound to human hemoglobin reveal the process of heme extraction

“…3 C ) comprises Hb residues belonging to A and H helices and the EF loop that interact directly or through bridging water with residues Phe164-Ala168 (loop 2), Gln190, Phe194, Phe242, and Asn243 on IsdB. Notably, the loop 2 contains the Hb-binding motif ( 3 , 18 ) and folds in a stable α-helix as also observed in the complex of Hb with isolated IsdB N1 and IsdH N1 domains ( 4 , 9 , 11 , 18 ). The folding upon binding of loop 2 is likely associated to a prolyl cis/trans isomerization (vide infra) and represents a driving force in stable complex formation.…”

“…IsdB is a modular protein formed by three domains: two NEAT (NEAr iron Transporter) domains (IsdB N1 and IsdB N2 ) separated by an intervening linker domain (IsdB L ) ( 9 ). NEAT domains present an immunoglobulin-like fold that binds heme and have been identified in many Gram + bacteria, with higher prevalence in pathogens ( 10 , 11 ). The two NEAT domains of IsdB play distinct roles in the process of heme extraction from Hb.…”

“…The two NEAT domains of IsdB play distinct roles in the process of heme extraction from Hb. IsdB N1 makes high-affinity binding to Hb through the surface-exposed loop 2 ( 11 ) and positions IsdB N2 for heme extraction. IsdB N2 carries the heme-binding motif ( 440 YDGQY 444 ) that is largely conserved in all NEAT domains of S. aureus ( 11 ).…”

“…IsdB N1 makes high-affinity binding to Hb through the surface-exposed loop 2 ( 11 ) and positions IsdB N2 for heme extraction. IsdB N2 carries the heme-binding motif ( 440 YDGQY 444 ) that is largely conserved in all NEAT domains of S. aureus ( 11 ). In the IsdB:heme complex Tyr440 in the motif coordinates the hemic iron and Tyr444 stabilizes the conformation of Tyr440 through H-bonding ( 12 ).…”

Directed inter-domain motions enable the IsdH Staphylococcus aureus receptor to rapidly extract heme from human hemoglobin

“…The heme is then pumped into the cytoplasm by the transporter, where it is degraded by oxygenases (IsdG and IsdI) to liberate its iron [16, 17]. The cell wall-associated proteins bind heme and Hb using NEAT (NEAr iron Transporter) domains, which are conserved in other Gram-positive bacteria [18, 19]. Understanding how heme is extracted from Hb could lead to novel treatments to combat lethal bacterial infections, as genetic elimination of genes encoding Isd proteins decreases S. aureus virulence, and other clinically important pathogens employ similar protein machinery to acquire heme during infections [13, 15, 20–23].…”