Abstract
Iron is an essential nutrient for the proliferation of Staphylococcus aureus during bacterial infections. The iron-regulated surface determinant (Isd) system of S. aureus transports and metabolizes iron porphyrin (heme) captured from the host organism. Transportation of heme across the thick cell wall of this bacterium requires multiple relay points. The mechanism by which heme is physically transferred between Isd transporters is largely unknown because of the transient nature of the interactions involved. Herein, we show that the IsdC transporter not only passes heme ligand to another class of Isd transporter, as previously known, but can also perform self-transfer reactions. IsdA shows a similar ability. A genetically encoded photoreactive probe was used to survey the regions of IsdC involved in self-dimerization. We propose an updated model that explicitly considers self-transfer reactions to explain heme delivery across the cell wall. An analogous photo-cross-linking strategy was employed to map transient interactions between IsdC and IsdE transporters. These experiments identified a key structural element involved in the rapid and specific transfer of heme from IsdC to IsdE. The resulting structural model was validated with a chimeric version of the homologous transporter IsdA. Overall, our results show that the ultra-weak interactions between Isd transporters are governed by bona fide protein structural motifs.
Highlights
iron-regulated surface determinant (Isd) proteins convey heme molecules across the bacterial cell wall by means of sequential and transient protein-protein complexes
Self-transfer Reaction in IsdA and IsdC—We demonstrated that the IsdA and IsdC transporters are capable of self-transfer reactions (Fig. 1)
This is a novel finding that expands our understanding of heme transport by the Isd system across the thick cell wall (ϳ30 nm) of S. aureus [28]
Summary
Isd proteins convey heme molecules across the bacterial cell wall by means of sequential and transient protein-protein complexes. Structural studies have showed that the IsdA and IsdC transporters, as well as the C-terminal domains of IsdH and IsdB, share a common heme-binding domain known as NEAT (near transporter) (supplemental Movie 1) [15–21]. This domain is characterized by a rather hydrophobic pocket and displays a conserved tyrosine residue that coordinates the metal atom of the porphyrin ring. A separate study has suggested that NEAT domains may undergo self-exchange heme transfer reactions, no experimental evidence has been reported [21] This hypothesis could explain heme relay across the entire length of the cell wall, estimated at ϳ30 nm [28], with only two classes of anchored transporters, IsdA and IsdC. Our results demonstrate that ultra-weak protein-protein interactions between Isd proteins are governed by specific structural motifs
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