A Deletion in the First Cysteine‐Rich Repeat of the Low‐Density‐Lipoprotein Receptor Leads to the Formation of Multiple Misfolded Isomers

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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 LBI 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 mutant 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.