Lessons on protein structure from interleukin-4: All disulfides are not created equal

Area of Science:

• Biochemistry
• Structural Biology
• Immunology

Background:

• Interleukin-4 (IL-4) is a vital hematopoietic cytokine involved in immune responses.
• IL-4's four-helix bundle structure is stabilized by three disulfide bonds.
• IL-4 serves as a model protein for design and engineering studies.

Purpose of the Study:

• To investigate the role of each disulfide bond in IL-4 structure and dynamics.
• To understand how disulfide bond disruption affects IL-4 stability and function.

Main Methods:

• Spectroscopic analyses including circular dichroism (CD), fluorescence, and nuclear magnetic resonance (NMR).
• Molecular dynamics (MD) simulations.
• Analysis of wild-type IL-4 and four disulfide mutants.

Main Results:

• All disulfide mutants exhibited structural destabilization, altered angles, and looser packing.
• Disulfide bond removal led to loss of secondary structure and increased protein dynamics.
• Mutants lacking one or two disulfide bonds showed significant structural perturbations, including molten globule formation.

Conclusions:

• All three disulfide bonds are essential for maintaining the overall fold and stability of the IL-4 four-helix bundle.
• Disulfide bond integrity is critical for IL-4's structural integrity and dynamics, even under varying pH conditions.