From Nanotubes to Disks:
How Droplets Remodel Cell Membranes
 

Biomolecular condensates may play a crucial but overlooked role in remodeling membrane structures within cells. Rumiana Dimova and her team demonstrated that these droplets can shape parts of the endoplasmic reticulum into nanotubes and double-membrane disks. Because of their wetting properties, condensates act like architects without the need for specific curvature-molding proteins, and understanding droplet-membrane interactions enables us to grasp the intricate mechanisms of cell organization.

Cells are the smallest units of life, and Rumiana Dimova's team is studying what happens inside their crowded and dynamic interiors. The team designed simplified artificial replicas called giant vesicles and filled them with water-soluble polymers that can separate into tiny droplets – much like oil separates from vinegar in a salad dressing. In cells, these tiny droplets are called biomolecular condensates, and they act as compartments performing specific functions.

These droplets may play a crucial role in remodeling cell membrane structures like the endoplasmic reticulum (ER), a network of membranes that acts as a conveyor belt for proteins. The ER consists of tubular and flat disk-like parts, and scientists believe it is proteins that keep them all in shape. But Dimova and her team observed something unexpected – these membranes did not need proteins to adjust their shape to the changing conditions.

To simulate the ER in the vesicles, the researchers removed water. At first, the membranes molded into nanotubes to preserve their surface. "As I removed more water, looking at them through a super-resolution microscope revealed that the nanotubes had transformed into double-membrane disks, similar to pancakes,” describes Dr. Ziliang Zhao. This transformation occurred when the nanotubes touched the droplets that mimicked the condensates.

“Beyond the well-known scaffolding proteins, droplets may also be important architects: cellular membranes change their shape while sticking to the condensates,” - emphasizes Dimova. Decoding these complex inner mechanisms is fundamental to our grasp of how cells function and organize, and it holds potential for the development of future methods to detect and understand captivating membrane behavior.