Conformation and lipid binding properties of peptide models of exchangeable apolipoproteins

Lipoproteins solubilize and transport lipids and cholesterol in the body. They contain proteins (apolipoproteins) that function structurally, as cofactors for enzymes, and in receptor binding. The exchangeable apolipoproteins traffic between lipoproteins and share a sequence pattern of two types (A and B) of eleven-residue amphipathic alpha-helical repeats, thought to be lipid-binding elements. Because of conformational flexibility of exchangeable apolipoproteins, peptide models have been used to study the properties of these repeats. Two overlapping 44-residue (ABAB) real sequence peptides (RSP-1, residues 99--142, and RSP-2, 121--163) from the sequence of apolipoprotein A-I and two 22-residue consensus sequence peptides (CSP-AB and CSP-BA), designed as organizational rearrangements of consensus A and B repeats, were studied. Peptide conformation and thermal stability were studied using circular dichroism and the lipid-binding properties of the RSPs were analyzed using density gradient sedimentation and electron microscopy. In phosphate buffer, RSP-1 was 23% helical while RSP-2 was unstructured even though they have 50% sequence overlap. Helical structure could be induced in both peptides (up to 75% for RSP-1 and 50% for RSP-2) in lipid-mimicking detergent or structure-promoting solvents. The secondary structure of RSP-1 was insensitive to pH, but RSP-2 showed a loss of helical content in detergent at acidic pH, possibly due to two histidine residues that lie in the hydrophobic face of its second AB repeat. Both peptides bound and solubilized phospholipid, with RSP-1 having higher affinity than RSP-2. The particles formed depended upon the peptide-to-lipid ratio and both vesicular and phospholipid bilayer discoidal particles, similar to the nascent form of HDL, were observed. Furthermore, RSP-1 formed more discoidal particles than RSP-2, consistent with their different lipid affinities. CSP-AB in phosphate buffer showed 26% helical structure that increased up to 50% in detergent or solvents, while CSP-BA was unstructured in phosphate buffer and could only be induced to adopt 30% helix. This work further elucidates the conformational and lipid-binding properties of the helical lipid-binding repeats of exchangeable apolipoproteins by defining the different lipid affinities of homologous repeated sequences and the effect of organizational arrangement of the repeats.