Vesicle fission of giant unilamellar vesicles of liquid-ordered-phase membranes induced by amphiphiles with a single long hydrocarbon chain.

Vesicle fissions are very important processes of biomembranes in cells, but their mechanisms are not clear and are controversial. Using the single giant unilamellar vesicle (GUV) method, we recently found that low concentrations (less than the critical micelle concentration (CMC)) of lysophosphatidylcholine (lyso-PC) induced the vesicle fission of GUVs of dipalmitoylphosphatidylcholine/cholesterol(6/4) (DPPC/chol(6/4)) membranes and sphingomyelin/cholesterol membranes (6/4) in the liquid-ordered (lo) phase. In this report, to elucidate its mechanism, we have investigated the effect of low concentrations (much less than their CMC) of other amphiphiles with a single long hydrocarbon chain (i.e., single long chain amphiphiles) on DPPC/chol(6/4) GUVs as well as the effect of the membrane composition on the lyso-PC-induced vesicle fission. We found that low concentrations of single long chain amphiphiles (lyosophosphatidic acid, octylglucoside, and sodium dodecyl sulfate) induced the shape change from a prolate to two spheres connected by a very narrow neck, indicating that the single long chain amphiphiles can be partitioned into the external monolayer in the lo phase of the GUV from the aqueous solution. As the single long chain amphiphile concentrations were increased, all of them induced vesicle fission of DPPC/chol(6/4) GUVs above their threshold concentrations. To elucidate the role of cholesterol in the single long chain amphiphile-induced vesicle fission, we investigated the effect of lyso-PC on GUVs of dioleoyl-PC (DOPC)/chol(6/4) membranes in the Lalpha phase; no vesicle fission occurred, indicating that cholesterol in itself did not play an important role in the vesicle fission. Finally, to elucidate the effect of the inclusion of DOPC in the lo-phase membrane of GUVs on the lyso-PC-induced vesicle fission of the DPPC/chol(6/4) GUV, we investigated the effect of low concentrations of lyso-PC on GUVs of DPPC/DOPC/chol membranes. With an increase in DOPC concentration in the membrane, the threshold concentration of lyso-PC increased. At and above 30 mol % DOPC, no vesicle fission occurred. On the basis of these results, we have proposed a hypothesis of the mechanism of the single long chain amphiphile-induced vesicle fission of a GUV of a lo-phase membrane.