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Proline isomerization modulates the bacterial IsdB/hemoglobin interaction: an atomic force spectroscopy study.

Francesca Pancrazi1, Omar De Bei2, Francesco Lavecchia di Tocco1

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Staphylococcus aureus surface protein IsdB exhibits catch bond behavior with host hemoglobin, increasing its interaction strength under mechanical stress. A Pro173 mutation disrupts this, revealing cis-trans isomerization as the molecular basis for stress-dependent binding in this bacterial virulence factor.

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Area of Science:

  • Microbiology
  • Structural Biology
  • Biophysics

Background:

  • Staphylococcus aureus (SA) utilizes Iron surface determinant B (IsdB) for heme iron acquisition and bacterial adhesion, acting as a key virulence factor.
  • IsdB exhibits catch bond behavior, strengthening its interaction with host cell adhesion factors and hemoglobin under mechanical stress, potentially aiding bacterial invasion.
  • The molecular mechanisms underlying this stress-dependent binding in IsdB are not fully understood.

Purpose of the Study:

  • To elucidate the structural and molecular basis for the catch bond behavior of IsdB when interacting with host hemoglobin.
  • To investigate the role of the Pro173 residue in the mechanical stress-dependent interaction of IsdB with hemoglobin.

Main Methods:

  • Single-molecule force spectroscopy (e.g., Atomic Force Spectroscopy) was used to analyze the IsdB:hemoglobin interaction under varying mechanical forces.
  • Site-directed mutagenesis was employed to create a Pro173 mutant of IsdB.
  • Structural analysis was performed to understand the conformational changes upon complex formation.

Main Results:

  • A single point mutation at Pro173 in the IsdB hemoglobin-binding domain abolished the catch bond behavior, weakening the IsdB:hemoglobin interaction.
  • Pro173 does not directly bind hemoglobin but undergoes cis-trans isomerization upon complex formation, coupled with folding-upon-binding of a protein loop.
  • These events involving Pro173 isomerization and loop folding are proposed as the molecular basis for the stress-dependent strength of the wild-type IsdB:hemoglobin interaction.

Conclusions:

  • The Pro173 residue and its associated cis-trans isomerization are critical for the catch bond behavior of IsdB with hemoglobin.
  • Understanding these molecular mechanisms provides insights into Staphylococcus aureus's host cell invasion strategies under mechanical stress.
  • This knowledge can inform the development of novel antibacterial strategies targeting IsdB function.