Mutation studies previously showed that the lentivirus lytic peptide (LLP2) sequence of the cytoplasmic C-terminal tail of the HIV-1 gp41 envelope protein inhibited viral-initiated T-cell death and T-cell syncytium formation, at which time in the HIV life cycle the gp41 protein is embedded in the T-cell membrane. In striking contrast, the mutants did not affect virion infectivity, during which time the gp41 protein is embedded in the HIV envelope membrane. To examine the role of LLP2/membrane interactions, we applied synchrotron x-radiation to determine structure of hydrated membranes. We focused on WT LLP2 peptide (+3 charge) and MX2 mutant (-1 charge) with membrane mimics for the T-cell and the HIV-1 membranes. To investigate the influence of electrostatics, cholesterol content, and peptide palmitoylation, we also studied three other LLP2 variants and HIV-1 mimics without negatively charged lipids or cholesterol as well as extracted HIV-1 lipids. All LLP2 peptides bound strongly to T-cell membrane mimics, as indicated by changes in membrane structure and bending. In contrast, none of the weakly bound LLP2 variants changed the HIV-1 membrane mimic structure or properties. This correlates well with, and provides a biophysical basis for, previously published results that reported lack of a mutant effect in HIV virion infectivity in contrast to an inhibitory effect in T-cell syncytium formation. It shows that interaction of LLP2 with the T-cell membrane modulates biological function.

determined the phase behavior of total synaptosomal lipids by repeating the NMR protocol on extracted lipids. To our surprise, there was no detectable signal for liquid-ordered lipid phases at the body temperature of either species. However, there was a difference in the temperature of the onset of order as synaptosomes were cooled below body temperature: the phase state of mouse synaptosomal membranes changes drastically below 24 C, whereas this change occurs below 8 C for the squid synaptosomal membranes. We then measured the composition of synaptosomes in terms of total lipid heads and tails, and the main difference arises from high concentrations of omega-3 poly-unsaturated fatty acids and cholesterol in the squid. Fluorescence microscopy images of squid synaptosomes stained with lipid dyes confirmed the formation of domains below, but not above the phase transition temperatures obtained from NMR measurements. Thus although the membranes of synaptosomes contain lipids that can phase-separate, these lipids remain in the liquid-disordered state at the usual physiological temperatures for squid and mouse.