Influenza Fusion Peptide and Transmembrane Domain Interaction Induces Distinct Domains in Lipid Bilayers

Abstract

Viral glycoproteins, such as influenza HA2 and HIV gp41, are anchored by a single helical segment transmembrane domain (TMD) on the viral envelope membrane. The function of the TMD's is still unclear, but it is speculated to be involved in the final steps of membrane fusion. The interactions between the influenza HA fusion peptide (FP) and the TMD, as well as the HIV gp41 FP and TMD have previously been shown. We found that the FP or TMD alone perturbs membrane structure. We hypothesized that the FP-TMD interaction will further change the membrane structure which provides a driving force for the fusion between the inner membranes. We used PC spin labels DPPTC (on the head group), 5PC and 14PC (5-C and 14-C positions on the acyl chain) to detect the perturbation by titrating the FP to TMD-reconstituted POPC/POPG/Chol lipid bilayers by ESR. We found that the FP increases the lipid order in all positions. In addition to the increase of lipid order, multiple components are found for 5PC when the concentration of FP increases, indicating that distinct domains are induced. The FP or TMD alone does not induce this change. Therefore, this domain formation should be an effect of FP-TMD interaction. Although HA-mediated membrane fusion is pH dependent, the domain formation is observed at both pH5 and pH7. The mutations on TMD do not affect the domain formation. However, the deletion of G1 in the FP eliminates all perturbations. We conclude that the influenza FP-TMD interaction induces distinct domains in the lipid bilayers, which is pH independent but relies on the glycine ridge in the N-terminal helix of the FP. We are extending our research on ITC and ESR on spin-labeled peptides to further characterize this interaction.