Apolipoprotein E4 forms a molten globule. A potential basis for its association with disease

Area of Science:

• Biochemistry
• Protein Folding
• Molecular Biology

Background:

• Apolipoprotein E (apoE) exists in three common isoforms: apoE2, apoE3, and apoE4.
• Isoform-specific differences in apoE structure and function are linked to various diseases, including Alzheimer's disease.
• Understanding the biophysical properties of apoE isoforms is crucial for elucidating their physiological and pathological roles.

Purpose of the Study:

• To compare the urea denaturation and folding intermediate formation of the amino-terminal domains of apoE isoforms (apoE2, apoE3, apoE4).
• To characterize the structural properties of the apoE4 folding intermediate.
• To investigate how differential folding propensities of apoE isoforms may contribute to disease.

Main Methods:

• Urea denaturation monitored by circular dichroism spectroscopy.
• Analysis of denaturation curves using two-state and three-state unfolding models.
• Structural characterization of the apoE4 folding intermediate using pepsin proteolysis, Fourier transform infrared spectroscopy, and dynamic light scattering.

Main Results:

• Apolipoprotein E4 (apoE4) and apoE3 exhibited distinct folding intermediates at pH 4.0, with apoE4 forming a more stable intermediate.
• Apolipoprotein E2 (apoE2) was the most stable isoform and did not display a distinct folding intermediate.
• The apoE4 folding intermediate was identified as a molten globule with a partially opened four-helix bundle, exposing its hydrophobic core.

Conclusions:

• ApoE4 is more susceptible to unfolding and prone to forming a stable molten globule intermediate compared to apoE3 and apoE2.
• The distinct molten globule formation propensity of apoE isoforms may underlie their differential roles in health and disease.
• These findings provide insights into the molecular mechanisms contributing to apoE isoform-specific disease pathologies.