The structural basis for insulin-like growth factor I receptor high affinity binding.

Area of Science:

• Biochemistry
• Molecular Biology
• Cell Signaling

Background:

• The insulin-like growth factor I (IGF I) receptor plays a critical role in cell growth and differentiation.
• Previous studies identified high and low affinity IGF I binding sites in human placental membranes.

Purpose of the Study:

• To investigate the structural basis of high-affinity IGF I binding by examining the effect of disulfide bond reduction on the IGF I receptor.
• To elucidate the role of receptor subunit association in IGF I binding affinity.

Main Methods:

• Purification of the high-affinity IGF I receptor using IGF I affinity chromatography.
• Incubation of the receptor with dithiothreitol (DTT) at pH 8.75 to reduce disulfide bonds.
• Analysis of receptor dissociation and IGF I binding parameters using gel filtration chromatography and Scatchard plots.

Main Results:

• Disulfide bonds linking the alpha beta dimers of the IGF I receptor heterotetramer were reduced by DTT treatment.
• Reduced alpha beta dimers dissociated and exhibited a 6.1-fold lower affinity for IGF I compared to the intact heterotetramer.
• The binding capacity increased 1.6-fold for the dissociated dimers, suggesting each dimer binds IGF I with lower affinity.

Conclusions:

• Association of two alpha beta dimers is essential for the formation of a high-affinity IGF I receptor.
• The heterotetrameric structure facilitates high-affinity IGF I binding, while dissociated dimers bind the ligand with lower affinity.