Abstract
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.
Highlights
Periodicity of amino acid arrangement to produce multiple amphipathic a-helixes is encoded into the genomic structure of these proteins [1]
In apolipoproteins A-I, A-IV and E, the ll-mer repeats have evolved into 22-mer tandem repeats, with most of these repeats having the periodicity of a class A amphipathic a-helix [2,3,4]
The formation of discoidal particles with phospholipid was explained on the basis of shielding of the hydrophobic face of the amphipathic helix at the edge of the lipid bilayer [6]. It follows that the relative areas of the hydrophilic and hydrophobic faces in the amphipathic helixes influence the lipid-associating properties of the exchangeable apolipoproteins
Summary
Vol 270, No., Issue of January 27, pp. 1602-1611, 1995 Printed in U.S.A. Effect of the Arrangement of Tandem Repeating Units of Class A Amphipathic a-Helixes on Lipid Interaction*. 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. We compare the lipid interactions of three synthetic peptides containing two tandem repeating units of class A amphipathic helical segments. The lipid-associating properties of the above two peptides have been compared with that of 36A (18A-18A), in which the linker residue (Pro in 37pA and Ala in 37aA) is deleted This peptide was designed to form a longer class A amphipathic a-helix in which the polar and nonpolar faces are aligned along the length of the entire molecule. The results of this study show that the degree of alignment of the faces of different tandem repeating amphipathic helical segments in a peptide can affect its lipidassociating properties
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