Transport-deficient Mutations in the Low Density Lipoprotein Receptor. Alterations in the Cysteine-rich and Cysteine-poor Regions of the Protein Block Intracellular Transport

Certain individuals with familial hypercholesterolemia (FH) produce mutant forms of the low density lipoprotein (LDL) receptor that fail to move from the endoplasmic reticulum to the Golgi complex. Here, we describe the cloning and expression of one such mutant allele, FH 429. The mutation causes a substitution of a Val for a Gly at residue 544. When recreated in an expressible cDNA, this substitution gives rise to an LDL receptor that is not transported to the cell surface and is rapidly degraded. Three previously mapped transport-deficient alleles of the LDL receptor were traced to the cysteine-rich repeats of the protein, suggesting that the generation of non-disulfide-bonded (free) cysteines might cause the block in transport. The FH 429 mutation is not located in a cysteine-rich region, however. We have attempted to test the role of cysteine by expressing mutant cDNAs that encode proteins blocked in transport and predicted to contain free cysteines. The results suggest that free cysteines are not obligatory for the blocked intracellular movement of mutant LDL receptors.