Interactions of the N-terminal domain of human islet amyloid polypeptide with lipid membranes: the effect of cholesterol

The 1-19 region of human islet amyloid polypeptide (hIAPP(1-19)) is a dominating factor causing the interaction between hIAPP and membrane. It contains a short sequence RLANFLV that fulfils the aminoacid arrangement of the inversed cholesterol recognition amino-acid consensus (CARC) and may mediate a direct contact of hIAPP with cholesterol. In this study, we focused on the interaction of hIAPP(1-19) with the lipid membrane composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and cholesterol, and examined the role of the CARC motif in the peptide-membrane interaction. Using differential scanning calorimetry, P-31-NMR spectroscopy, H-1-NMR titration measurement and dye leakage assay, we demonstrated that hIAPP(1-19) interacts with DPPC vesicles more strongly in the presence of cholesterol than it does in the absence of cholesterol. The peptide-membrane interaction promotes the domain segregation of the raft-containing membrane. The peptide is more disruptive to the cholesterol-containing membrane than it is to the cholesterol-depleted membrane. The substitution of the residue Phe at position 15 of hIAPP(1-19) by Leu leads to a distinct decrease in the peptide-membrane interaction in the presence of cholesterol, but the effect of the residue substitution on the peptide-membrane interaction is very small in the absence of cholesterol. The circular dichroism data indicated that a conversion of the structure from a random coil to an a-helix is induced by cholesterol for both peptides and the structural conversion is more Chol-dependent for the wild-type peptide than the F15L variant. Our findings suggest that cholesterol could facilitate the insertion and aggregation of the N-terminal domain of hIAPP in the membrane, and the phenylalanine in the CARC motif could be involved in the interaction of the N-terminal domain with Chol.