Lateral Phase Separation And Budding In Multicomponent Lipid Vesicles

Lipid bilayer membranes are ubiquitous in biological systems, which exhibit rich shape transition behavior, and are closely related with cell functions, such as lipid transport, exocytosis, endocytosis, signaling and so on. Budding is always the first and important step in all these processes. To understand the mechanism of these biological processes, investigations of the budding on the multicomponent lipid vesicles have become a hot topic in membrane biology and biophysics. In this thesis, we focus on the study of the lateral phase separation and budding dynamics on the multicomponent lipid vesicles. The main results are summarized in the following:1. We have designed and setup two systems, which enable to prepare the samples in situ, and to study in real time the phase separation, budding, and its coalescence under the microscope.2. The growth of a single bud has been studied in real time under the microscope. It is found when the bud develops to a flat-bottom half sphere and reaches to its maximum size, it does not form a complete sphere on the membrane as predicted theoretically by Lipowsky et al., but continue to grow up to a tubular-shape bud with the fixed limiting radius. This new result can be interpreted by a model calculation based on a simple thermodynamic argument. The coalescence among buds and domains are also investigated in detail. The results show that the domains and buds diffuse in membrane via the aggregation of molecules, and the coalescence depends on both the bending rigidity and the bud (domain) diffusion coefficient. In addition, it is also observed that two tubular buds coalesce each other with an angle from their roots.3. For the budding dynamics of multicomponent unilamellar vesicles, it is found that the shape deformations are different for the spherical vesicles with smaller area-to-volume ratio and the tubular vesicles with larger area-to-volume ratio. The non-sphere shape vesicles are necessary for the formation of large number of buds, the number of buds on tubular vesicle as a function of growth time with a scaling law of N_bud～t^-2/3 has been determined, which agrees quite well with the theoretical works by Kumar et al and Laradji et al. Another interesting observation is that the packing of domains and buds on a large spherical surface obeys the crystallography on a sphere.4. Tight coiling single and double helical vesicles are observed in both DPPC/DOPC/Chol and BSM/DOPC/Chol lipid mixtures. Under some controlled perturbations, the single helix could be stretched smoothly and recovers automatically like a spring. In addition, the soliton-like coiling defects are observed to move along the helical structure, which seems to serve as the trigger of instability of helical vesicles.5. The phase separation and shape deformation of multilamellar vesicles in DPPC/DOPC/Chol mixture are investigated for the first time. It is found that the formation of domains on each lamellar is independent to each other at the initial stage, then domains start to move on the membrane spontaneously and line up along the radial direction. It is found that such a phase rearrangement is corresponds to the shape deformation.6. Phase separation and shape deformation of unilamellar and double lamellar vesicles are studies using the discrete-spatial variation method. It is found and discussed in details that the line tension, pressure difference, bending rigidity and spontaneous curvature are all important for the budding process, while the space between two lamellas in double lamellar vesicles are critical for the phase rearrangement.