The Effects of Bilayer Sidedness and Flip-Flop of Lysophosphatidylcholine on Viral Fusion with Model and Biological Systems

Abstract

Intermediate lipid structures such as inverted micelles and interlamellar attachment are thought to play a crucial role in different biological processes like viral infection. Lysophosphatidylcholine has been shown to inhibit membrane fusion at stabilizing concentrations (between 1 and 10% with respect to membrane lipids). Studies in this thesis looked at the effects of Lysophosphatidylcholine (LPC) properties on the inhibition of Sendai viral fusion. The effects of bilayer sidedness preference as well as flip-flop of Lysophosphatidylcholine (lyso PC) were examined. Octadecylrhodamine (R₁₈) lipid mixing assays were used to measure the fusion of Sendai virus with different biological, erythrocyte ghosts, and artificial systems consisting of different lipids and different viral receptor compositions. The data showed that external addition of LPC exhibits a dependency between the incubation time of the lysolipids and the inhibition of viral fusion. The results also demonstrate a relationship between the location of LPC in the bilayer and its ability to inhibit lipid mixing. LPC present only on the outer monolayer plays a role in the inhibition of viral fusion. Reorientation of LPC was also measured for the same incubation periods. A method using Bovine Serum Albumin (BSA) and radioactivity labelled LPC, was applied to measure the flip-flop. Significant transbilayer reorientation of Lyso PC in the bilayer was shown to take place. The rate of flip-flop was measured at 0.32 ± 0.08% LPC /min. Such reorientation can explain the time dependency observed earlier. The conclusions of this thesis lend support to stalk intermediate mechanism of viral membrane fusion. The ability of LPC to inhibit only when present on one side of the bilayer supports the idea of a negatively curved stalk intermediate. Moreover, it showed that the shape and curvature tendencies of the bilayer stabilizer determine its effects on viral fusion.