Apolipoprotein E3-Leiden contains a seven-amino acid insertion that is a tandem repeat of residues 121-127.
Apolipoprotein (apo) E3-Leiden is a variant of apoE that is associated with dominant expression of type III hyperlipoproteinemia and that is defective in binding to the low density lipoprotein receptor. Therefore, the structure of apoE3-Leiden was investigated. Upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis apoE3-Leiden and its 22-kDa amino-terminal thrombolytic fragment migrated with a higher than normal apparent molecular weight. The structural abnormality of apoE3-Leiden was determined by sequencing its CNBr-, tryptic-, and Staphylococcus aureus V8 protease-generated peptides. In contrast to normal apoE3, which has a cysteine at residue 112, apoE3-Leiden does not contain any cysteine and has an arginine at position 112 (as does apoE4, which also completely lacks cysteine). The basis for the molecular weight difference was determined to be a seven-amino acid insertion that is a tandem repeat of residues 121-127 of normal apoE3, i.e. Glu-Val-Gln-Ala-Met-Leu-Gly, resulting in apoE3-Leiden having 306 amino acids rather than 299. The negatively charged glutamyl residues within the insertion compensates for the arginine substitution at residue 112; thus apoE3-Leiden focuses in the E3 position. The low density lipoprotein receptor binding activities of both intact apoE3-Leiden and its 22-kDa thrombolytic fragment were determined in an in vitro assay. Although apoE3-Leiden had only about 25% of normal binding activity, its 22-kDa thrombolytic fragment had nearly normal binding, suggesting that the carboxyl-terminal domain of apoE3-Leiden modulates the receptor binding function of its amino-terminal domain.