Cell membranes transition seamlessly between different three-dimensional configurations. It is a remarkable feature that is essential for many biological phenomena such as cell division, cell motility, transport of nutrients into cells, and viral infections. Researchers at the Indian Institute of Science (IISc) and their collaborators recently devised an experiment that sheds light on the mechanism by which such processes might occur in real time.
The researchers looked at colloidal films, which are micrometer-thick layers of aligned rod-like particles. Colloidal membranes provide a more attractive system to study, as they exhibit many of the same properties as cell membranes. Unlike a plastic sheet, where all the molecules are immobile, cell membranes are fluid sheets in which every component is free to diffuse. “This is a key property of cell membranes that is available in ours [colloidal membrane] system as well,” explains Prerna Sharma, Associate Professor in the Department of Physics, IISc, and corresponding author of the study published in the journal Proceedings of the National Academy of Sciences.
The colloidal films were synthesized by preparing a solution of rod viruses of two different lengths: 1.2 µm and 0.88 µm. The researchers studied how the shape of colloidal films changes as the fraction of short rods in solution increases. “I made multiple samples by mixing different volumes of the two viruses and then observed them under a microscope,” explains Ayantika Khanra, Ph.D. student in the Department of Physics and the first author of the paper.
Image (false color) of a fluidized colloidal membrane self-assembled from a binary mixture of short and long rods. Credit: Ayantika Khanra
When the proportion of short rods increased from 15% to between 20–35%, the films switched from a flat disk shape to a saddle-like shape. Over time, the membranes began to merge and increase in size. The saddles were ranked by their order, which is the number of ups and downs one encounters as one moves along the edge of the saddle. The researchers observed that when the saddles joined laterally, they formed a larger saddle of the same or higher order. However, when they merged at an almost right angle, away from their edges, the final configuration was a catenoid-like shape. Catenoids then fused with other saddles, creating increasingly complex structures such as trinoids and tetranoids.
To explain the observed behavior of the membranes, researchers have also proposed a theoretical model. According to the laws of thermodynamics, all physical systems tend to move toward low energy configurations. For example, a drop of water takes on a spherical shape because it has less energy. For films, this means that shapes with smaller edges, such as a flat disc, are more preferred. Another property that plays a role in determining membrane conformation is the Gaussian curvature coefficient. A key insight of the study was to show that the Gaussian curvature coefficient of membranes increases when the fraction of short rods increases. This explains why adding more short rods drove the films into saddle-like shapes, which have lower energy. It also explains another observation from their experiment where the low-order films were small in size, while the high-order films were large.
“We have proposed a new mechanism for creating curvature of fluidic membranes. This mechanism of regulating curvature by changing the Gaussian coefficient could be at play in biological membranes as well,” says Sharma. He adds that they want to continue studying how other microscopic changes in membrane components affect the large-scale properties of membranes.
Combustion of membranes for molecular sieving More information: Ayantika Khanra et al, Controlling the shape and topology of two-component colloidal membranes, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2204453119 Provided by Indian Institute of Science
Reference: Three-dimensional configuration of microscopic membranes underlying cellular processes (2022, September 12) Retrieved September 12, 2022, from
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