An allosteric mechanism controls antigen presentation by the H-2K(b) complex

Area of Science:

• Immunology
• Molecular Biology
• Biochemistry

Background:

• Major Histocompatibility Complex (MHC) class I molecules present peptide antigens to T cells.
• Understanding the assembly dynamics of MHC class I molecules is crucial for immune response.
• Recombinant, water-soluble MHC class I molecules provide a model for studying these dynamics.

Purpose of the Study:

• To elucidate the assembly and dissociation mechanism of the recombinant H-2Kb molecule.
• To investigate the allosteric regulation of interactions between MHC heavy chain, beta2-microglobulin (beta2m), and antigenic peptides.
• To compare the binding kinetics and stability with related MHC class I molecules.

Main Methods:

• Real-time fluorescence resonance energy transfer (FRET) was employed to monitor molecular interactions.
• Kinetic analysis was used to determine rate constants for association and dissociation.
• Biexponential kinetics were analyzed to understand complex formation and conformational changes.

Main Results:

• An allosteric mechanism controls the interactions among the H-2Kb heavy chain, beta2m, and peptides.
• Association with beta2m significantly enhances peptide binding rates and affinity for the heavy chain.
• Peptide binding increases the affinity of the heavy chain for beta2m, indicating a cooperative interaction.
• Ternary complex formation and dissociation exhibit biphasic kinetics, suggesting conformational flexibility.
• Dissociation rates of beta2m were peptide-independent, while peptide dissociation varied.
• H-2Kb/peptide complexes showed greater stability than H-2Kd counterparts.

Conclusions:

• The H-2Kb molecule assembly is regulated by an allosteric mechanism involving sequential binding events.
• The molecule can exist in multiple conformations, influencing its stability and peptide interactions.
• These findings provide insights into the dynamic nature of MHC class I peptide loading and presentation.