Folding determinants of LDL receptor type A modules

Area of Science:

• Biochemistry
• Structural Biology
• Molecular Biology

Background:

• The low-density lipoprotein receptor (LDLR) plays a vital role in cholesterol homeostasis.
• The ligand-binding module 5 (LR5) of the LDLR is critical for its function.
• Protein structure is stabilized by disulfide bonds and metal ion coordination.

Purpose of the Study:

• To investigate the cooperative roles of disulfide bonds and calcium ion coordination in specifying the structure of the LDLR-A module (LR5).
• To understand the interdependence of disulfide bond formation and calcium coordination during LR5 folding.

Main Methods:

• Site-directed mutagenesis to create LR5 variants with altered cysteine pairs.
• Nuclear Magnetic Resonance (NMR) spectroscopy to probe protein conformation.
• Biochemical assays to assess disulfide bond formation and calcium binding.

Main Results:

• Single disulfide bonds in LR5 do not induce calcium-dependent structural changes.
• The presence of two C-terminal disulfide bonds is required for calcium-dependent native disulfide bond formation and structural changes in LR5.
• Calcium coordination primarily stabilizes the C-terminal lobe of LR5, leaving the N-terminal lobe disordered.

Conclusions:

• Cooperation between specific disulfide bonds and calcium ion coordination is essential for the proper folding and structural integrity of the LDLR-A module.
• Unlike other model proteins, LR5 folding is not driven by a single disulfide bond but requires a specific arrangement of two disulfide bonds and calcium.
• These findings highlight the complex structural requirements for LDLR function and provide insights into protein folding mechanisms.