Dynamic conformations compared for IgE and IgG1 in solution and bound to receptors

Area of Science:

• Immunology
• Biophysics
• Structural Biology

Background:

• Immunoglobulin (Ig) isotypes like IgE and IgG1 play critical roles in immune responses.
• Understanding the dynamic conformations of Ig molecules is essential for elucidating their function.
• Previous studies suggested bent conformations for IgE, but IgG1 dynamics remained less understood.

Purpose of the Study:

• To investigate and compare the dynamic conformations of murine IgE and human IgG1.
• To determine the end-to-end distances and flexibility of IgE and IgG1 in solution and when bound to their respective receptors.
• To assess whether IgG1 also adopts a bent conformation, similar to IgE.

Main Methods:

• Utilized fluorescence resonance energy transfer (FRET) measurements with specific donor and acceptor probes attached to the N-terminal (Fab) and C-terminal (Fc) ends of Ig molecules.
• Studied IgE and IgG1 in solution and when bound to their high-affinity receptors: Fc epsilon RI on RBL cell membranes and Fc gamma RI on U937 cell membranes.
• Employed a collisional quencher to assess segmental flexibility and extract Gaussian distributions of end-to-end distances.

Main Results:

• IgE in solution showed an average end-to-end distance (rho) of 75 Å with a half-width (hw) of 24 Å, and when bound to Fc epsilon RI, rho=71 Å and hw=12 Å.
• IgG1 in solution exhibited an average end-to-end distance of 75 Å.
• IgG1 bound to Fc gamma RI displayed an average end-to-end distance greater than or equal to 85 Å, with a broad distribution (rho=100 Å, hw=88 Å), indicating a significantly bent conformation.

Conclusions:

• Both IgE and IgG1 adopt bent conformations, deviating from a planar Y-shape, in solution and when engaged with their receptors.
• The binding of IgG1 to its receptor (Fc gamma RI) appears to induce a more pronounced bent conformation compared to its solution state.
• These conformational dynamics are crucial for the diverse functions of different immunoglobulin isotypes.