E. Gazit et al., STRUCTURE AND ORIENTATION OF THE MAMMALIAN ANTIBACTERIAL PEPTIDE CECROPIN P1 WITHIN PHOSPHOLIPID-MEMBRANES, Journal of Molecular Biology, 258(5), 1996, pp. 860-870
Cecropins are positively charged antibacterial peptides that act by pe
rmeating the membrane of susceptible bacteria. To gain insight into th
e mechanism of membrane permeation, the secondary structure and the or
ientation within phospholipid membranes of the mammalian cecropin P1 (
CecP) was studied using attenuated total reflectance Fourier-transform
infrared (ATR-FTIR) spectroscopy and molecular dynamics simulations.
The shape and frequency of the amide I and II absorption peaks of CecP
within acidic PE/PG multibilayers (phosphatidylethanolamine/phosphati
dylglycerol) in a 7:3 (w/w) ratio (a phospholipid composition similar
to that of many bacterial membranes), indicated that the peptide is pr
edominantly alpha-helical. Polarized ATR-FTIR spectroscopy was used to
determine the orientation of the peptide relative to the bilayer norm
al of phospholipid multibilayers. The ATR dichroic ratio of the amide
I band of CecP peptide reconstituted into oriented PE/PG phospholipid
membranes indicated that the peptide is preferentially oriented nearly
parallel to the surface of the lipid membranes. A similar secondary s
tructure and orientation were found when zwitterionic phosphatidylchol
ine phospholipids were used. The incorporation of CecP did not signifi
cantly change the order parameters of the acyl chains of the multibila
yer, further suggesting that CecP does not penetrate the hydrocarbon c
ore of the membranes. Molecular dynamics simulations were used to gain
insight into possible effects of transmembrane potential on the orien
tation of CecP relative to the membrane. The simulations arrear to con
firm that CecP adopts an orientation parallel to the membrane surface
and does not insert into the bilayer in response to a cis positive tra
nsmembrane voltage difference. Taken together, the results further sup
port a ''carpet-like'' mechanism, rather than the formation of transme
mbrane pores, as the mode of action of CecP According to this model, f
ormation of a layer of peptide monomers on the membrane surface destab
lizes the phospholipid packing of the membrane leading to its eventual
disintegration. (C) 1996 Academic Press Limited