Aggregation of transmembrane peptides studied by spin-label EPR.

The structure and function of membrane proteins is partly determined by the interaction of these proteins with the lipids of the membrane. Peptides forming single membrane-spanning alpha-helices, such as the WALP peptide (acetyl-GWWL(AL)(n)WWA-amide), are good models for such interactions. This interaction can be studied by investigating the aggregation of peptides. If the peptides remain isolated in the membrane, the peptide-lipid interaction dominates, if the peptides aggregate, the peptide-peptide interaction is stronger. The intrinsic dynamics and the disordered nature of the system require new approaches to determine eventual aggregation. We performed electron paramagnetic resonance (EPR) on spin-labeled WALP (SL-WALP) in the gel and the liquid-crystalline phases of two different phospholipids, the saturated DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine), and the unsaturated DOPC (1,2-dioleoyl-sn-glycero-3-phosphocholine). At low temperatures (120 K) where both lipids are in the gel phase, less extensive aggregation is observed for the peptide in DOPC as compared to DPPC. Together with previous data on aggregation of WALP peptides from atomic force microscopy and fluorescence spectroscopy at 294 K ( Sparr ; et al. Biochemistry 2005 , 44 , 2 -10 ), the results suggest that at 120 K WALP peptides form line aggregates in DOPC and cluster aggregates in DPPC. In the liquid-crystalline phase of both lipids, signatures of aggregation are absent, showing that in this phase the peptide can be accommodated by either lipid. It can be concluded that the lipid phase, in this case gel or liquid-crystalline, is a more important determinant for peptide aggregation than whether the lipid is saturated (DPPC) or unsaturated (DOPC). In view of the gel-phase-like behavior of some membrane phases in physiological systems the methodology should be relevant.