Structural characterization of two peptides corresponding to Helix 4 of Apolipoprotein E

Abstract

Apolipoprotein E (ApoE) is a ligand for the low density lipoprotein (LDL) receptor. The amino-terminal domain of ApoE is a 4-helix bundle and helix 4 is involved in LDL receptor binding. This region associates with lipids and undergoes structural changes which allow a key arginine residue, Arg172 to participate in receptor binding. To characterize this region and to study lipid-induced conformational changes we developed 2 synthetic peptides: one corresponding to residues 128-164 (ApoE-164) and the other corresponding to residues 128-183 (ApoE-183). Secondary structure prediction indicates amphipathic alpha-helical structure for both peptides. Circular dichroism spectroscopy (CD) studies show that ApoE-164 exhibits concentration-dependent self association whereas ApoE-183 does not. To study the effect of lipids on the two peptides we used three detergents: 1,2-diheptanoyl-sn-glycero-3-phosphocholine (DHPC), dodecyl phosphocholine(DPC), and octyl-beta-d-glucopyranoside(BetaOG). CD shows that both peptides exhibit an increase in alpha-helix content at the critical micelle concentration (CMC) for all three detergents. The alpha-helix content for both peptides in the absence of detergent is about 30%. Upon the addition of DHPC, DPC or BetaOG there is a 30-40% increase in alpha-helicity at the CMC. This approach is directed towards understanding (i) how lipids induce a conformational change in the 165-183 region, and (ii) how the relative orientation of Arg172 is changed such that it contributes to LDL receptor binding.