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Organelles are functional subunits of eukaryotic cells. Most of organelles are bounded by lipid bilayer membrane (membrane-bound organelle), inside of which have different chemical conditions from cytoplasm to carry out specialized biochemical reactions. Organelles are identified by their characteristic shapes as well as their biological functions. However, no valid mechanisms to generate and maintain their shapes have been proposed. In this study, we demonstrate a process to generate membrane-bond organelle by physical model simulations of lipid membrane system and propose a possible mechanism to produce the characteristic shape.

We consider Golgi apparatus which is a membrane-bound organelle to work as the hub of the cellular logistics. Golgi apparatus consists of several flattened of membrane sacs (cisternae) which are stacked to each other. In the course of cell division, Golgi apparatus disassemble into small vesicles which are partitioned into both daughter cells. These vesicles re-assemble and fuse each other to generate the Golgi shape at the end of the cell division. This re-assembly process is highly dynamics process to produce complex Golgi shapes from assembly of simple vesicles. Thus, in this study, we aim to reproduce this re-assembly process and reveal the mechanism underlying it.

In the simulations, arbitrary shape of curved membrane surface is approximately represented by a triangulated polygon, and the temporal shape change is performed using Monte Carlo method, in which deformations by force relaxation and thermal fluctuation are both represented following the statistical mechanics. The relation between local shapes of bilayer lipid membrane and its deformation energy is well characterized by elastic membrane theory, where the bending energy is the square of curvature of membrane.

To represent cisternae stacking, vesicle membrane has attractive force field to adhere each other. When vesicles come close to each other, they fuse into a larger membrane structure which promotes re-assembly. Referring in vivo vesicle behaviors, small vesicles are added in a interval into a small region in simulations. First we found that above basic set-up simulations without any additional assumption did not give Golgi-like shapes, but gave sponge-like shapes. These shapes were found to be caused by the excess of membrane fusion. In particular, no limitation in membrane fusion destroys the already formed layered structure. We then gave an additional assumption of the limitation of membrane fusion: fusion probability depends on the membrane curvatures and less curved membrane has lower probability to fuse. This assumption is biologically relevant if the distribution of fusion machinery depends on the membrane curvature. With the additional assumption we obtained the nicely layered structures of membrane. We consider this assumption is one of the most simple mechanism to give the Golgi-like structure. Moreover, this simplicity will give a hint to discuss the generation and maintenance of Golgi apparatus in the evolutionary history.

modeling, simulation