The interactions of the N-terminal fusogenic peptide of HIV-1 gp41 with neutral phospholipids

We have studied the interactions with neutral phospholipid bilayers of FPI, the 23-residue fusogenic N-terminal peptide of the HIV-I-LAI transmembrane glycoprotein gp41, by CD, EPR, NMR, and solid state NMR (SSNMR) with the o bjective of understanding how it lyses and fuses cells. Using small unilame llar vesicles made from egg yolk phoshatidylcholine which were not fused or permeabilised by the peptide we obtained results suggesting that it was ca pable of inserting as an a-helix into neutral phospholipid bilayers but was only completely monomeric at peptide/lipid (P/L) ratios of 1/2000 or lower . Above this value, mixed populations of monomeric and multimeric forms wer e found with the proportion of multimer increasing proportionally to P/L, a s calculated from studies on the interaction between the peptide and spin-l abelled phospholipid. The CD data indicated that, at P/L between 1/200 and 1/100, approximately 68% of the peptide appeared to be in a-helical form. W hen P/L=1/25 the alpha-helical content had decreased to 41%. Measurement at a P/L of 1/100 of the spin lattice relaxation effect on the C-13 nuclei of the phospholipid acyl chains of an N-terminal spin label attached to the p eptide showed that most of the peptide N-termini were located in the interi or hydrocarbon region of the membrane. SSNMR on multilayers of ditetradecyl phosphatidyl choline at P/Ls of 1/10, 1/20 and 1/30 showed that the peptide formed multimers that affected the motion of the lipid chains and disrupte d the lipid alignment. We suggest that these aggregates may be relevant to the membrane-fusing and lytic activities of FPI and that they are worthy of further study.