Corynebacterium diphtheriae Iron-Regulated Surface Protein HbpA Is Involved in the Utilization of the Hemoglobin-Haptoglobin Complex as an Iron Source

Abstract

utilizes various heme-containing proteins, including hemoglobin (Hb) and the hemoglobin-haptoglobin complex (Hb-Hp), as iron sources during growth in iron-depleted environments. The ability to utilize Hb-Hp as an iron source requires the surface anchored proteins HtaA and either ChtA or ChtC. The ability to bind hemin, Hb and Hb-Hp by each of these proteins requires the previously characterized Conserved Region (CR) domain. In this study, we identified an Hb-Hp binding protein, HbpA (38.5-kDa), which is involved in the acquisition of hemin-iron from Hb-Hp. HbpA was initially identified from total cell lysates as an iron-regulated protein that binds to both Hb and Hb-Hp HbpA does not contain a CR domain and has sequence similarity only to homologous proteins present in a limited number of strains. Transcription of is regulated in an iron-dependent manner that is mediated by DtxR, a global iron-dependent regulator. Deletion of from results in reduced ability to utilize Hb-Hp as an iron source, but has little or no effect on the ability to use Hb or hemin as iron sources. Cell fractionation studies showed that HbpA is both secreted into the culture supernatant and associated with the membrane where its exposure on the bacterial surface allows HbpA to bind Hb and Hb-Hp. The identification and analysis of HbpA enhances our understanding of iron uptake in , and indicates that the acquisition of hemin-iron from Hb-Hp may involve a complex mechanism that requires multiple surface proteins.The ability to utilize host iron sources, such as heme and heme-containing proteins, is essential for many bacterial pathogens to cause disease. In this study, we have identified a novel factor (HbpA) that is crucial for the use of hemin-iron from the hemoglobin-haptoglobin complex (Hb-Hp). Hb-Hp is considered one of the primary sources of iron for certain bacterial pathogens. HbpA has no similarity to the previously identified Hb-Hp binding proteins, HtaA and ChtA/C, and is only found in a limited group of strains. Understanding the function of HbpA may significantly increase our knowledge of how this important human pathogen can acquire host iron that allows it to survive and cause disease in the human respiratory tract.

Pirates of the haemoglobin

“…Indeed, HbpA deletion mutants in iron depleted media showed reduced ability to use the Hb:Hp complex as an iron source, while showing no difference in the use of Hb or haem. [37] In summary, HtaA, HtaB, ChtA and ChtC are surface exposed proteins of C. diphtheriae and are able to bind host haemoproteins, starting the cascade of haem-passing interactions that follow; for example, HtaA to HtaB to HmuT then onto HmuUV for transmembrane transportation. The ability to use Hb as an iron source is not unique within the genus to C. diphtheriae.…”

Heme acquisition and tolerance in Gram-positive model bacteria: An orchestrated balance

“…In addition to the heme-binding proteins mentioned above, a unique iron-regulated, Hb- and Hb-Hp binding protein (HbpA) was also identified in C. diphtheriae [ 40 ]. HbpA is both located in the membrane and secreted, and exists in a large-molecular-weight aggregate that is important for optimal binding to Hb and Hb-Hp [ 41 ].…”

“…HbpA is both located in the membrane and secreted, and exists in a large-molecular-weight aggregate that is important for optimal binding to Hb and Hb-Hp [ 41 ]. HbpA does not bind heme and contains a CR domain but can bind Hb and Hb-Hp [ 40 ]. Structural analysis of HbpA protein revealed that the C-terminal region is pivotal for the use of heme-iron from Hb-Hp and plays a role in anchoring to the membrane [ 41 ].…”

“…Structural analysis of HbpA protein revealed that the C-terminal region is pivotal for the use of heme-iron from Hb-Hp and plays a role in anchoring to the membrane [ 41 ]. Homologues of HbpA are only distributed in some of C. diphtheriae strains [ 40 ].…”

“…The heme acquisition system of C. diphtheriae acquires heme through the following process. At the first step, most Hb and Hb-Hp are bound by secreted or membrane-associated HbpA, and HbpA can promote the binding of Hb-Hp to surface receptors HtaA and ChtA/C [ 40 ]. HtaA and ChtA/C bind heme from Hb or Hb-Hp via CR domains and transport heme to HtaB or ChtB [ 34 , 41 ].…”