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A research team has gained new insights into the molecular processes of clathrin-mediated endocytosis—the central process by which cells take up nutrients, messenger substances and receptors.

The study by junior research group leader Dr. Sigrid Milles, Andromachi Papagiannoula and Dr. Ida Marie Vedel from the Leibniz-Forschungsinstitut für Molekulare Pharmakologie in Berlin focuses on the molecular interactions between the EH domains of the endocytosis protein Eps15 and an intrinsically disordered fragment of Dab2.

The results are in Nature Communications.

Using high-resolution NMR spectroscopy, the researchers were able to show that the EH domains of Eps15 bind with much more versatility than previously assumed. Instead of recognizing only one classic binding motif, they also attach promiscuously to other phenylalanine-containing sequences.

Surprisingly, Eps15 not only binds to its partner Dab2, but also to its own intrinsically disordered segment (Eps15IDR). This leads to a kind of self-inhibition, but at the same time ensures that Eps15 can form liquid-like condensates (liquid-liquid phase separation). Dab2320-495 alone cannot form droplets, but is recruited into the Eps15 condensates without significantly disrupting their formation.

An exciting finding of the study is that both Eps15IDR and Dab2320-495 can bind to the combined domain region EH123 at the same time, even though they compete for the same binding pockets. This interaction creates a very dynamic, flexible network.

"The fact that proteins that compete with the interaction between EH domains and Eps15IDR can penetrate the Eps15 condensates is astonishing and was not directly expected, but it means that Eps15 is probably capable of recruiting many other proteins to the membrane," says Sigrid Milles.

Weak bonds as a prerequisite for cell flexibility

The results show that weakness is a strength here: the bonds between the proteins are not rigid, but deliberately loose. Only in this way can they quickly detach and reattach—a decisive factor for the high dynamics of endocytosis.

The researchers led by Sigrid Milles have thus provided a new model for how molecular versatility and self-organization ensure that vital transport processes in the cell run smoothly and flexibly.

In the long term, this knowledge could help to better understand diseases in which endocytosis is disrupted, such as cancer or neurodegenerative diseases.