A research team led by Rumiana Dimova at the Max Planck Institute of Colloids and Interfaces has devised a mechanism to produce consistent nanotubes without applying pressure on the membrane that works without needing motor proteins.
It is based on osmosis. If more molecules are found outside the cell creating a hypertonic solution, water will flow from the cell and it will shrink. The team replicated the concentration differences with simulated vesicles as small as a cell, comprising a mixture of two polymers, polyethylene glycol (PEG) and dextran. Biopolymers exist in identical concentration in living organisms. The team moved the vesicle to a hypertonic solution, making it emit water and contract in volume.
The water caused the dissolved polymers in the vesicle to rise. This caused the polymers to split. So, two separate droplets developed in the vesicle like a snowman with one big circle of PEG molecules and a small one of dextran molecules. A fluorescence microscope helped the team notice that membrane nanotubes developed in the PEG-dominated region and gathered in the gap between the two drops. A model determined the stability of the nanotubes, which showed them that when the polymers are split solution flows of varying densities are caused, exerting pressure on the membrane leading to development of the tubes.
The membrane shapes showed that stable tubes developed only when two sides of the membrane were asymmetrical. This happens when the membrane and biopolymers interact with each other. PEG molecules accumulate on one side, while none exist on the other. The PEG communicates with the lipid molecules in the membrane so that it tries to curl inwards. The nanotube developed has this feature of the cellular membrane. If the vesicle inflates via osmosis, the nanotubes disappeared.