There has been a new cell organelle that has been found! It's called the hemifusome. The hemifusome deals with the recycling pathway inside the cell. This organelle was found by scientists at the University of Virginia. They were able to identify this brand new organelle with the help of cryo-electron tomography. This type of imaging freezes cells and creates three-dimesnsional images of their internal structures.
The hemifusome is made of two different types of vesicles that are hemifused together. The larger vesicle is granular, while the smaller one has a smooth lumen. They create a hemifusion diaphragm, which is a continuous bilayer connecting them. The diaphragm is not small: the diameter is about 158.4 nm based on the topomograms, while the larger portions are about 299 nm in diameter. At the rim of the hemifusome, there is a 42 nm proteolipid nanodroplet that is embedded in the membrane. This droplet likes to be in the hydrophobic core of the bilayer. This droplet also had a particulate structure (proteins) on the cytoplasmic side. It is interpreted that this droplet is at this junction to change the lipid packaging properties at the hemifusome diaphragm to lower the energy barrier for the formation of vesicles and budding. There are two different conformations in this organelle that exist in the periphery of the cell. The direct hemifusome is when the smaller vesicle is on the cytoplasmic side, and the flipped hemifusome is when the smaller vesicle is more internal. This organelle makes up about 10 percent of the vesicular organelles in the cell.
These hemifusomes play an integral part in the cell function. They essentially are early assembly hubs for generating internal vesicles that end up in late endosomes. A good metaphor for this action is thinking of the hemifusome as a loading dock. The delivery trucks (vesicles) pull up, connect, unload, or load the cargo, and then leave or merge. These vesicles will connect to the hemifusome diaphragm. By using the proteolipid nanodroplet, the hemifusome could be used to sort and process both lipids and proteins. The droplets' consistent location at the diaphragm suggests that it plays a critical role in this sorting mechanism, rather than being a random placement
The discovery of this new recycling and sorting mechanism matters because many diseases develop when cells can't properly organize or move their intracellular cargo. The researchers point out a strong connection to Hermansky-Pudlak syndrome, a disorder where the defects in vesicle trafficking lead to problems with pigmentation, bleeding, and organelle function. This finding helps explain how those trafficking errors might occur in the first place. It also raises bigger questions about how this pathway works in healthy versus damaged cells, and whether targeting it in the future could help correct these issues or even serve as a future therapeutic approach.
Image of a hemifusome:
Wow what a cool discovery! Who knew that the lysosome had a cousin? I wonder if this new imaging technique will uncover additional organelles in the future. The possible link to Hermansky-Pudlak syndrome is interesting, do you think defects in the hemifusome itself could someday be used as a diagnostic marker?
ReplyDeleteFascinating how we've studied cells for ages, and bam… out pops the hemifusome, this fused vesicle setup running the cell's recycling show. What grabs me is that proteolipid droplet at the edge, quietly bending the rules of membrane physics to make budding happen smoothly. Keeps you humble, wondering what other surprises are hiding in plain sight.
ReplyDeleteReference:
Tavakoli, A., Hu, S., Ebrahim, S., & Kachar, B. (2025). Discovery of the hemifusome, a novel organelle involved in vesicle formation and cellular cargo processing. Nature Communications. https://doi.org/10.1038/s41467-025-59887-9