Shape Deformations of Multi-Component Vesicles
Y. Sakuma and M. ImaiOchanomizu University, Japan
Deformations of cell membranes, such as fusion, adhesion, budding, and pore formation, play importantroles in the maintenance of living cell systems. In cell systems, the membrane deformations are managed bycomplex interplays between membrane proteins and lipids. It is hypothesized that one role of the membraneproteins is to introduce a local spontaneous curvature by interacting with the membranes [1], although therelationship between the local spontaneous curvature and the shape deformations has not been demonstrated.
In this context, we have been investigating the shape deformations of giant unilamellar vesicles (GUVs)
induced by the local spontaneous curvature of the membranes [2-4].
We have introduced the local spontaneous curvature in the membrane by coupling spontaneouscurvatures of lipids with a phase separation. For binary membranes composed of immiscible lipids having
different geometries, the local spontaneous curvature and the bending modulus depend on the localcomposition of the membrane. The heterogeneity of membrane coupled with the line energy at the phaseboundary brings new shape deformation branches. For example, the binary GUV composed of theinverse-cone-shaped lipids and cylinder-shaped lipids showed adhesion through the domains rich ininverse-cone-shaped lipids by forming an hourglass-like interbilayer structure called a stalk. A fluorescentlipid transfer experiment strongly implied that a stalk formed between adhering vesicles [2]. On the other
hand, in the binary vesicle composed of cone- and cylinder-shaped lipids, the main chain transition ofcone-shaped lipids reduces the surface area, which results in the pore formation. The segregated cone-shapedlipids form the cap at the edge of bilayer and the cylinder-shaped lipid in gel state solidifies the main body ofthe GUV. These effects stabilize the pore below the main chain transition temperature. By the pore formation,
the binary GUVs showed different morphologies at the rim of the pore (smooth-, rolled-, and wrinkled-rim)
depending on the concentration of the cone-shaped lipid, indicating the asymmetric distribution of thecone-shaped lipid in the bilayer. The exchange of lipids through the capped rim and the coupling of the local
composition of membrane with the membrane curvature brings a local asymmetry in composition between
the inner and outer leaflet, which causes vesicle shape deformations, such as the formation of rolled-rim.
We believe that our results provide new insight into the physical basis of the shape deformation ofcellular membranes.
REFERENCES
1. Zimmerberg, J., and Kozlov, M., ”How proteins produce cellular membrane curvature” Nature Rev. Mol.
Cell. Biol. 7,9-19, (2006).
2. Sakuma, Y., Imai, M. Yanagisawa, M. and Komura, S., “Adhesion of binary giant vesicles containing
negative spontaneous curvature lipids induced by phase separation” Eur. Phys. J. E. 25, 403-413, (2008).
3. Yanagisawa, M, Imai, M. and Taniguchi, T., “Shape deformation of ternary vesicles coupled with phaseseparation” Phys Rev Lett 100, 148102, (2008).
4. Sakuma, Y., Taniguchi, T. and Imai, M., “Pore Formation in a Binary Giant Vesicle Induced byCone-Shaped Lipids”, Biophys. J. in press.

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