Effect of the arrangement of tandem repeating units of class A amphipathic alpha-helixes on lipid interaction.

Exchangeable apolipoproteins possess tandem repeating units of class A amphipathic helical segments and many of them are linked together by proline residues. To understand the optimal arrangement of the amphipathic helixes for lipid association, we have studied the interactions of three model class A amphipathic helical peptides with lipids. The three peptides are: 37pA, a dimer of 18A (DWLKAFYDKVAEKLKEAF) linked together by a Pro (18A-Pro-18A); 37aA, a dimer of 18A linked together by an Ala (18A-Ala-18A); and 36A, a dimer of 18A without any linker residue (18A-18A). Circular dichroism (CD) spectra showed that the peptides are predominantly alpha-helical in aqueous and lipid environments. Temperature dependent CD studies indicated that in buffer helix stability decreases in the order 36A > 37aA > 37pA; however, in the presence of dimyristoyl phosphatidylcholine (DMPC), the above order is reversed. The retention times of the peptides on a C18 reversed-phase high performance liquid chromatography column decreased in the order 36A > 37aA > 37pA, consistent with the lengths of the nonpolar faces of the alpha-helixes being in the same order; the retention time of the parent 18A was shorter than 37pA. While 37pA adsorbed to egg phosphatidylcholine monolayers most strongly, the degree and rate of association of 36A were significantly lower. Differential scanning calorimetry indicated that, while 37pA was most effective in reducing the enthalpy of the gel to liquid-crystalline phase transition of DMPC multilamellar vesicles, 36A was least effective; 36A was even less effective than 18A. Fluorescence quenching experiments with iodide and acrylamide indicated that, in the presence of DMPC, Trp residues in 36A are most exposed to the quenchers while in 37pA they are least exposed. In the presence of DMPC, shielding of Trp in 18A from the quenchers was more than that observed with Trp residues in 36A. The results of this study suggest that the arrangement of tandem repeating amphipathic helical units which results in the formation of a class A amphipathic helix with a nonpolar face longer than five or six turns reduces the ability of the helix to associate with phospholipid.