MspA Porin-Gold Nanoparticle Assemblies: Enhanced Binding through a Controlled Cysteine Mutation

Area of Science:

• Biophysics
• Nanotechnology
• Microbiology

Background:

• Mycobacterial porin A (MspA) forms an octameric channel in Mycobacterium smegmatis.
• Gold nanoparticles (AuNPs) are widely used in biomedical applications due to their unique optical properties.
• Understanding nanoparticle-protein interactions is crucial for developing targeted drug delivery and diagnostic tools.

Purpose of the Study:

• To investigate the interactions between MspA and AuNPs of different sizes.
• To explore the role of cysteine residues in MspA-AuNP binding.
• To characterize the binding affinity and proximity of AuNPs to MspA.

Main Methods:

• Photoluminescence quenching assays to detect AuNP-MspA interactions.
• Fluorescence resonance energy transfer (FRET) to assess proximity.
• High-performance liquid chromatography (HPLC) to determine binding constants.

Main Results:

• Small AuNPs (3.7 nm) showed enhanced photoluminescence quenching with the MspA Q126C mutant, indicating binding within the pore.
• Large AuNPs (17 nm) did not exhibit quenching, consistent with size exclusion.
• Energy transfer was observed between small AuNPs and MspA, confirming close proximity, but was absent for large AuNPs.
• The MspA Q126C mutant demonstrated significantly higher binding affinity (>10^12 M^-1, irreversible) for small AuNPs compared to wild-type MspA (1.3 x 10^9 M^-1).

Conclusions:

• The eight cysteine residues in the MspA Q126C mutant enhance the binding of small gold nanoparticles within the MspA pore.
• Size plays a critical role in AuNP-MspA complex formation, with larger nanoparticles excluded from the pore.
• Fluorescence-based methods effectively probe AuNP-protein interactions and proximity, providing insights into binding mechanisms.