Nanotube formation in a vesicle containing two droplets (PEG - dark, and dextran - green). The membrane is labelled in red. After deflation of the vesicle, nanotubes form within the PEG-rich phase and accumulate at the interface between the two droplets. (a-c) Vertical cross sections of the vesicle; (d) top view of the nanotubes located at the interface. © Max Planck Institute of Colloids and Interfaces
Tubular membrane structures can be found in many areas of a cell: in the Golgi apparatus, a type of sorting station in which transport vesicles are formed; in the mitochondria, the power plants of the cell; or in the endoplasmic reticulum, a type of duct network within cells. The tubes have a diameter ranging from a few nanometres (one millionth of a millimetre) to a few micrometres (one thousandth of a millimetre).
The thinner the tubes, the greater the surface to volume ratio. They are therefore ideal for storing a lot of membrane in rather small spaces. Researchers believe that motor proteins can use energy to pull nanotubes from cellular membranes. “However, motor proteins are not always found in the areas of the cell where membrane nanotubes are formed,” says Rumiana Dimova, a researcher at the Max Planck Institute of Colloids and Interfaces and co-author of the study. For this reason, she believes that there must be another mechanism to generate stable nanotubes.
The Potsdam-based researchers may have now found the answer to the riddle. “The mechanism generates stable nanotubes without forces having to be exerted on the membrane. It therefore seems to work without the need for motor proteins,” says Dimova. Part of the mechanism is based on a phenomenon that is omnipresent in the world of membranes, the so-called osmosis. If certain molecules are present in a larger concentration outside the cell than inside the cell – i.e. they form a so-called hypertonic solution – then water will flow out of the cell and the cell will contract.
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