Human low density lipoprotein receptor fragment. Successful refolding of a functionally active ligand-binding domain produced in Escherichia coli.

The low density lipoprotein (LDL) receptor plays a key role in cholesterol homeostasis, mediating cellular uptake of lipoprotein particles by high affinity binding to its ligands, apolipoprotein (apo) B-100 and apoE. The ligand-binding domain of the LDL receptor contains 7 cysteine-rich repeats of approximately 40 amino acids; each repeat contains 6 cysteines, which form 3 intra-repeat disulfide bonds. As a first step toward determining the structure of the LDL receptor, both free and bound to its ligands, we produced in Escherichia coli a soluble fragment containing the ligand-binding domain (residues 1-292) as a thrombin-cleavable, heat-stable thioredoxin fusion. Modest amounts (5 mg/liter) of partially purified but inactive fragment were obtained after cell lysis, heat treatment, thrombin cleavage, and gel filtration under denaturing conditions. We were able to refold the receptor fragment to an active conformation with approximately 10% efficiency. The active fragment was isolated and purified with an LDL affinity column. The refolded receptor fragment was homogeneous, as determined by sodium dodecyl sulfate or non-denaturing polyacrylamide gel electrophoresis and isoelectric focusing. The purified fragment did not react with fluorescein-5-maleimide, indicating that all 42 cysteines were disulfide linked. In addition, the refolded fragment exhibited properties identical to those of the intact native receptor: Ca2+-dependent binding and isoform-dependent apoE binding (apoE2 binding <5% of apoE3). Furthermore, antibodies to the fragment recognized native receptors and inhibited the binding of 125I-LDL to fibroblast LDL receptors. We conclude that we have produced a properly folded and fully active receptor fragment that can be used for further structural studies.