Beijing, October 9, 2024 – A groundbreaking study from the laboratory of Dr. Xiaofeng Fang at School of Life Sciences, Tsinghua University, in collaboration with international partners, was published today in Nature, unveiling a novel mechanism for membrane shaping and scission in multivesicular bodies (MVBs), which play a crucial role in cellular quality control. The study, titled "Biomolecular condensates mediate bending and scission of endosome membranes", demonstrates that biomolecular condensates formed by the plant protein FREE1 can mediate membrane invagination and scission without the need for ESCRT machinery or ATP consumption (Figure 1).
Figure 1. A model of ESCRT machinery-dependent and FREE1 condensate (magenta)-mediated ILV formation pathways from MVB membranes (green). (i) membrane invagination, (ii) membrane scission. Blue Y shapes indicate ubiquitinated cargoes.
Traditionally, the formation of intraluminal vesicles (ILVs) within MVBs has been thought to require energy-driven processes involving the ESCRT protein complex. However, this study reveals that FREE1, through phase separation into liquid-like condensates, can induce membrane bending and instability, facilitating the formation of ILVs in an energy-independent manner. This discovery marks a significant departure from the classical understanding of membrane dynamics.
Using a phase separation screening system developed in their lab, Dr. Fang's team identified FREE1 as a protein capable of undergoing strong phase separation both in vivo and in vitro. The research also showed that the N-terminal intrinsically disordered region (IDR) of FREE1 drives this phase separation. Through its FYVE domain, FREE1 binds to the membrane lipid phosphatidylinositol 3-phosphate (PI3P), anchoring the condensates to the MVB membrane. Importantly, the formation of these condensates enhances membrane binding and recruits other ESCRT components as client molecules.
The team’s experiments further demonstrated that replacing FREE1’s IDR with the IDR of the FUS protein could rescue the lethal phenotype of FREE1-deficient mutants, confirming that phase separation is necessary and sufficient for FREE1's function. However, this substitution did not enable interaction with ESCRT proteins, suggesting that FREE1’s condensates can function independently of the ESCRT machinery.
In vitro reconstruction and computer simulations revealed that FREE1 condensates rapidly induce membrane curvature and invagination. These findings were supported by genetic evidence in plants lacking ESCRT, where overexpression of FUS-IDR-FREE1 was sufficient to generate ILVs within MVBs. The study also highlighted the evolutionary significance of both ESCRT-dependent and FREE1-condensate mechanisms, which likely provide a dual mechanism for MVB biogenesis, enabling cells to adapt to environmental stress.
This research opens new avenues for understanding membrane dynamics and cellular quality control mechanisms, potentially impacting a wide range of biological and medical fields. This work was introduced as the Research Briefing in the same issue of Nature.
Link: https://www.nature.com/articles/s41586-024-07990-0
