A structural transition in class II major histocompatibility complex proteins at mildly acidic pH

Area of Science:

• Immunology
• Structural Biology
• Biochemistry

Background:

• Peptide binding to class II major histocompatibility complex (MHC) proteins is critical for adaptive immunity.
• Enhanced peptide loading by class II MHC molecules occurs at the acidic pH found in endosomal compartments.
• Previous studies proposed a low-pH-induced conformational change in class II MHC, but direct structural evidence was lacking.

Purpose of the Study:

• To provide direct evidence for a structural change in class II MHC proteins at mildly acidic pH.
• To investigate the reversibility of pH-induced conformational changes.
• To correlate structural transitions with peptide binding pH optima.

Main Methods:

• Assessing class II MHC conformation using susceptibility to denaturation by sodium dodecyl sulfate (SDS) and heat at varying pH.
• Utilizing the environment-sensitive fluorescent probe 8-anilino-1-naphthalene-sulfonic acid (ANS) to detect changes in protein hydrophobicity.
• Comparing pH-dependent structural transitions across different class II MHC alleles (IAd, IEd, IEk, IAk).

Main Results:

• Class II MHC molecules exhibited increased sensitivity to denaturation at pH 5.5-6.5, indicating a structural transition.
• The conformational change was fully reversible upon neutralization of acidic pH.
• ANS fluorescence revealed increased exposed hydrophobicity at mildly acidic pH, confirming a structural shift.
• Allele-specific differences in transition pH (e.g., IAd vs. IEd, IEk, IAk) were observed, correlating with known peptide binding pH optima.

Conclusions:

• pH directly influences the structure of class II MHC molecules.
• A reversible conformational change at acidic pH underlies the enhanced peptide binding observed in endosomal compartments.
• Allelic variations in pH-dependent structural transitions contribute to differential peptide binding characteristics.