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 amphipa thic helixes for lipid association, we have studied the interactions o f three model class A amphipathic helical peptides with lipids. The th ree peptides are: 37pA, a dimer of 18A (DWLKAFYDKVAEKLKEAF) linked tog ether 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 resid ue (18A-18A). Circular dichroism (CD) spectra showed that the peptides are predominantly alpha-helical in aqueous and lipid environments. Te mperature dependent CD studies indicated that in buffer helix stabilit y decreases in the order 36A > 37aA > 37pA; however, in the presence o f dimyristoyl phosphatidylcholine (DMPC), the above order is reversed. The retention times of the peptides on a C-18 reversed-phase high per formance liquid chromatography column decreased in the order 36A > 37a A > 37pA, consistent with the lengths of the nonpolar faces of the alp ha-helixes being in the same order; the retention time of the parent 1 8A was shorter than 37pA. While 37pA adsorbed to egg phosphatidylcholi ne 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 g el to liquid-crystalline phase transition of DMPC multilamellar vesicl es, 36A was least effective; 36A was even less effective than 18A. Flu orescence quenching experiments with iodide and acrylamide indicated t hat, 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 o bserved with Trp residues in 36A. The results of this study suggest th at the arrangement of tandem repeating amphipathic helical units which results in the formation of a class A amphipathic helix with a nonpol ar face longer than five or six turns reduces the ability of the helix to associate with phospholipid.