Human apolipoprotein E7: lysine mutations in the carboxy-terminal domain are directly responsible for preferential binding to very low density lipoproteins

Apolipoprotein E7 (apoE7) (apoE3 E244K/E245K) is a naturally occurring muta nt in humans that is associated with increased plasma lipid levels and acce lerated atherosclerosis, It is reported to display defective binding to low density lipoprotein (LDL) receptors, high affinity binding for heparin, an d like apoE4, preferential association with very low density lipoproteins ( VLDL), There are two potential explanations for the preference of apoE7 for VLDL: lysine mutations, which occur in the major lipid-binding region (res idues 244-272) of the carboxy-terminal domain of apoE7, could either direct ly determine the lipoprotein-binding preference or could interact with nega tively charged residues in the amino-terminal domain, resulting in a domain interaction similar to that in apoE4 (interaction of Arg-61 with Glu-255), which is responsible for the apoE4 VLDL preference. To distinguish between these possibilities, we determined the binding preferences of recombinant apoE7 and two amino-terminal domain mutants, apoE7 (E49Q/E50Q) and apoE7 (D 65N/E66Q), to VLDL-like emulsion particles, ApoE7 and both mutants displaye d a higher preference for the emulsion particles than did apoE3, indicating that the carboxy-terminal lysine mutations in apoE7 are directly responsib le for its preference for VLDL, Supporting this conclusion, the carboxy-ter minal domain 12-kDa fragment of apoE7 (residues 192-299) displayed a higher preference for VLDL emulsions than did the wild-type fragment, In addition , lipid-free apoE7 had a higher affinity for heparin than did apoE, However , when apoE7 was complexed with dimyristoylphosphatidylcholine or VLDL emul sions, the affinity difference was eliminated. In contrast to previous stud ies, we found that apoE7 does not bind defectively to the LDL receptor, as determined in both cell culture and solid-phase assays. We conclude that th e two additional lysine residues in the carboxy-terminal domain of apoE7 di rectly alter its lipid- and heparin-binding affinities. These characteristi cs of apoE7 could contribute to its association with increased plasma lipid levels and atherosclerosis.