Scientists have finally solved the decades-old puzzle of how the "protein machine" that allows cells to swallow molecules during endocytosis works.
Endocytosis is the process in which cells consume molecules so they can perform different function internally, the European Molecular Biology Laboratory (EMBL) reported. In this process, dimples are formed in the cell membrane which eventually seal up and create spherical vesicle within the cell. A lattice-like protein shell is formed on the surface of the vesicle, but until now researchers were unsure why this feature was important in the process.
Two models were created in the past to explain this phenomenon: in the first, the coat (called the clathrin lattice) first assembles as a flat structure and then bends to fit the vesicle; in the second, the clathrin lattice directly assembles into the shape of the membrane as it is drawn inwards. It has been widely accepted that the firs scenario is correct, but this new research suggests the second hypothesis is actually more accurate.
In the recent experiments, the researchers looked at human cell lines in which the sites where endocytosis took place had been tagged with a fluorescent marker. The team then used electron microscopy to image these sites and analyzed how their shapes changed over time.
The findings revealed he surface area of the clathrin coat remained unchanged during endocytosis, and only its curvature was altered as it was drawn inwards.
"Our results were surprising, because the proteins have to undergo some complicated geometric transformations to go from a flat to a curved shape, which is why the second model was favoured by scientists for such a long time," said John Briggs, senior scientist at EMBL Heidelberg.
In the future, the researchers hope to investigate the details of how this rearrangement occurs.
"We also want to look at other aspects of the process, such as how the molecules ingested by the cells might themselves influence the action of the clathrin proteins. Answering these fundamental questions of cell biology will help scientists better understand the whole process of endocytosis," said Marko Kaksonen, of EMBL Heidelberg.
The findings were published in a recent edition of the journal Science.
