Human apolipoprotein E N-terminal domain displacement of apolipophorin IIIfrom insect low density lipophorin creates a receptor-competent hybrid lipoprotein

The surface of Manduca sexta low density lipophorin (LDLp) particles was em ployed as a template to examine the relative lipid binding affinity of the 22 kDa receptor binding domain (residues 1-183) of human apolipoprotein E3 (apo E3). Isolated LDLp was incubated with exogenous apolipoprotein and, fo llowing re-isolation by density gradient ultracentrifugation, particle apol ipoprotein content was determined. Incubation of recombinant human apo E3(1 -183) with LDLp resulted in a saturable displacement of apolipophorin III ( apo Lp-III) from the particle surface, creating a hybrid apo E3(1-183)-LDLp . Although subsequent incubation with excess exogenous apo Lp-III failed to reverse the process, human apolipoprotein A-I (apo A-I) effectively displa ced apo E3(1-183) from the particle surface. We conclude that human apo E N -terminal domain possesses a higher intrinsic lipid binding affinity than a po Lp-III but has a lower affinity than human apo A-I. The apo E3(1-183)-LD Lp hybrid was competent to bind to the low density lipoprotein receptor on cultured fibroblasts. The system described is useful for characterizing the relative lipid binding affinities of wild type and mutant exchangeable apo lipoproteins and evaluation of their biological properties when associated with the surface of a spherical lipoprotein. (C) 1999 Elsevier Science Inc. All rights reserved.