A deletion in the first cysteine-rich repeat of the low-density-lipoprotein receptor leads to the formation of multiple misfolded isomers

Abstract

The ligand-binding domain of the low-density-lipoprotein (LDL) receptor comprises seven cysteine-rich repeats, each approximately 40 amino acids long. The deletion of two amino acids (Asp26 and Gly27) from the first of these repeats (LB1), leads to a defective LDL receptor, and the clinical syndrome of familial hypercholesterolemia [Leitersdorf, E., Hobbs, H. H., Fourie, A. M., Jacobs, M., van der Westhuyzen, D.R. & Coetzee, G.A. (1988) Proc. Natl Acad. Sci. USA 85, 7912-7916]. Receptors which reach the cell surface fail to bind IgG-C7, a conformation-specific monoclonal antibody directed to LB1. To determine the effects of the two-amino-acid deletion on the folding of the LB1 of the LDL receptor, we have expressed LB1 and the mutant repeat, des-Asp26, Gly27-LB1, as recombinant (rLB1 and des-Asp26, Gly27-rLB1) peptides, and have determined their ability to fold in vitro. Unlike rLB1, which folded into a single isomer that was recognized by IgG-C7 and had three disulfide bonds, des-Asp26, Gly27-rLB1 folded into an equilibrium mixture of four isomers. Each of these isomers contained three disulfide bonds, but none were recognized by IgG-C7. We suggest that LDL receptors in the endoplasmic reticulum (ER) of the cell also fold into an equilibrium mixture of distinct receptor molecules, each with an abnormally folded isomer of des-Asp26, Gly27-LB1, and that the retarded transport of receptors to the cell surface arises because only a subset of the isomers reaches the cell surface.