APOLIPOPROTEIN E5 (GLU(212)-]LYS) - INCREASED BINDING TO CELL-SURFACEPROTEOGLYCANS BUT DECREASED UPTAKE AND LYSOSOMAL DEGRADATION IN CULTURED FIBROBLASTS

A new apolipoprotein (apo) E variant, apoE5 (Glu(212) --> Lys) was ide ntified in a Turkish family. The variant was due to a point mutation ( GAG --> AAG) at the first nucleotide position of the codon encoding am ino acid residue 212 of the mature apoE. The 23-year-old index patient was heterozygous for the mutation. Examination of the proband's kindr ed revealed six heterozygous and two homozygous mutation carriers. Com pared to non-carriers, carriers of the mutation had slightly higher tr iglycerides (1.25 versus 1.11 g/l) and lower HDL cholesterol (0.36 ver sus 0.41 g/l). Very low density lipoproteins (VLDL) from an apoE5 (Glu (212) --> Lys) homozygote displayed enhanced binding (+17%, P < 0.05), but decreased uptake (-35%, P < 0.0001) and degradation (-51%, P < 0. 0001) in cultured fibroblasts, compared to E3/3-VLDL. The region of th e apoE molecule surrounding residue 212 contains a heparin binding dom ain. Consistently, the enhanced cell surface binding of E5/5-VLDL was observed in ''wild-type'' Chinese hamster ovary cells (+19%, P < 0.05) , but not in proteoglycan-deficient cells. The binding of E5/5-VLDL to heparin was increased (+22%, P < 0.05). As the endocytosis of apoE-co ntaining particles involves the transfer of proteoglycan-bound ligands to lipoprotein receptors, the stronger binding of apoE5 (Glu(212) --> Lys) to proteoglycans could reduce the rate at which the mutant is fi nally delivered to endocytotic pathways. These data may provide eviden ce for a functionally important heparin binding site around amino acid residue 212 of the apoE molecule in vivo.