Proto cell

1.Protocell
2.Shape deformation of vesicle induced by chemical stimuli
3.Three dimensional analysis of lipid vesicle shape
4.Shape deformation of multi-component vesicle
5.Slow Dynamics of nano-meter-sized domains on the membrane

Protocell

Synthesizing simple living cells that contain the minimal elements necessary to perform the basic functions of life will provide insight into the biophysical origins of life. In order to reveal physical basis of the basic functions of cellular life, we have developed simple model vesicle systems having membrane traffic and self-reproduction abilities. A key concept of this model self-reproducing vesicle system is the coupling of the main-chain transition and the shape of lipids, which controls the reduced volume of the vesicle and the local curvature of the membrane. To date we have succeeded to reproduce adhesion, pore formation and self-reproduction functions by simple binary vesicles.
[Related papers]
Adhesion of Binary Giant Vesicles Containing Negative Spontaneous Curvature Lipids
Induced by Phase Separation
Y. Sakuma, M. Imai, M. Yanagisawa, S. Komura
European Physical Journal E, 25, 403-413, (2008).
Pore Formation in a Binary Giant Vesicle Induced by Cone-Shaped Lipids
Y. Sakuma, T. Taniguchi, and M. Imai
Biophys. J. 99, 472-479 (2010).
Model System of Self-Reproducing Vesicles
Yuka Sakuma and Masayuki Imai
Phys. Rev. Lett. 107, 198101(1-5) (2011).

Shape deformation of vesicle induced by chemical stimuli

In the cellular life, the functionalities are triggered by chemical stimuli. For instance, in various types of migrating cells, an actively generated intracellular pH gradient exists along the axis of movement. The local pH is also of great importance in dynamic invaginations called cristae in mitochondria. We have investigated the chemically triggered shaped deformation of lipid vesicles using a microinjecting technique. We believe that studying the response of a biomimetic membrane to a local modification of its environment will help to shed light onto the relation between cellular phenomena and small-scale environment changes.
[Related papers]
Lipid membrane deformation in response to a local pH modification: theory and experiments
A.-F. Bitbol, N. Puff, Yuka Sakuma, Masayuki Imai, Jean-Baptiste Fournier, and Miglena I. Angelova
Soft Matter, 8, 6073-6082 (2012).

Three dimensional analysis of lipid vesicle shape

Analysis and visualization of the shape of complex microscopic 3D biological objects is a difficult task. The standard 2D optical or electron microscopy provides an image of the cross-section of the object that may not capture many of its features. We have developed and tested a new experimental methodology to analyze the shape of lipid vesicles. 3D imaging is instrumental in the analysis of nonaxisymmetric shapes and we used it to systematically explore the phase diagram, which agrees well with the prediction of the area-difference elasticity model.
[Related papers]
Three-Dimensional Analysis of Lipid Vesicle Transformations
A. Sakashita, N. Urakami, P. Ziherl, and M. Imai
Soft Matter, 8, 8525-8796 (2012).

Shape deformation of multi-component vesicle

One of the most fascinating properties of lipid membranes is that they easily deform their shapes according to external circumstances, which gives them the basic physical
aspects of biomembrane functionalities. For example, an astonishing parade of shape deformations in giant unilamellar vesicles (GUVs) is induced by the osmotic pressure difference. Another interesting feature is that biomembranes inherently comprise a multicomponent system and the constituents form heterogeneities on the membranes. For example, GUVs composed of saturated phospholipids, unsaturated phospholipids, and Chol exhibit lateral phase separations between the liquid-ordered phase and the Ld liquid-disordered phase. We have investigated shape transition kinetics coupled with phase separation.
[Related papers]
Growth Dynamics of Domains in Ternary Fluid Vesicles
M. Yanagisawa, M. Imai, T. Masui, S. Komura and Takao Ohta
Biophysical J. 92, 115-125 (2007).
Shape deformation of ternary vesicles coupled with phase separation
M. Yanagisawa, M. Imai, and T. Taniguchi
Phys. Rev. Lett. 100, 148102(1-4) (2008).
Periodic modulation of tubular vesicles induced by phase separation
M. Yanagisawa, M. Imai, and T. Taniguchi
Phys. Rev. E 82 051928(1-9) (2010).

Slow Dynamics of nano-meter-sized domains on the membrane

In bio-membranes, assemblies of phospholipids, cholesterol and specific proteins migrate in the sea of lipids. The lateral diffusion of these peptides, proteins, protein complexes and lipid rafts governs the chemical reactions among them, and hence their biological functions. Since the diffusion of the constituents embedded in the cell membrane is a complicated problem, the first approach is to consider the dynamical nature of Brownian objects in a two-dimensional fluid sandwiched by three-dimensional bulk water. We have investigated the diffusion coefficient of nanometer-sized liquid ordered domains on vesicles using small angle neutron scattering and neutron spin echo spectroscopy.
[Related papers]
Nano-meter-sized domain formation in lipid membranes observed by small angle neutron scattering
T. Masui, N. Urakami and M. Imai
Eur. Phys. J. E 27, 379-389 (2008)
Diffusion of domains on nanometer sized vesicle
Y. Sakuma, N. Urakami, Y. Ogata, M. Nagao, S. Komura, T. Kawakatsu and M. Imai
J. Phys. Conf. Ser. 251, 012036(1-4) (2010).
Hydrodynamic effects on concentration fluctuations in multicomponent membranes
S. Ramachandran, S. Komura, K. Seki and M. Imai
Soft Matter 7, 1524-1531 (2011).