# The Liquid State of RIM1α and RBP Condensates is Maintained by Lipids

## TL;DR

This study shows that RIM1α and RBP proteins form liquid condensates at synapses, and lipids help prevent these condensates from turning solid, which could lead to dysfunction.

## Contribution

The study reveals that lipids suppress the liquid-to-solid transition of RIM1α/RBP condensates, maintaining their liquid state under physiological conditions.

## Key findings

- RIM1α and RBP can spontaneously form biomolecular condensates under physiological conditions.
- These condensates can transition from liquid to solid over time, forming β-sheet-rich fibrillar aggregates.
- Lipid vesicles prevent the liquid-to-solid transition, preserving the liquid state of RIM1α/RBP condensates.

## Abstract

At the presynapse, RIM1α and RIM-binding protein
(RBP) play
a crucial role in regulating vesicle docking and priming. Emerging
evidence suggests that these proteins are involved in the organization
of the active zone where they cluster with membrane proteins and lipids,
forming protein condensates driven by liquid–liquid phase separation
(LLPS). While protein phase separation has been associated with cellular
function, it has also been linked to various disorders, as liquid
condensates can promote protein aggregation, leading to dysfunction.
In this work, we investigated the phase behavior of RIM1α and
RBP. We find that under physiological conditions and in the absence
of crowding agents, RIM1α and RBP have the ability to spontaneously
form biomolecular condensates. Moreover, these liquid condensates
have the propensity to mature over time, resulting in a liquid-to-solid
transition. Using a combination of fluorescence microscopy with biophysical
techniques and characterization methods, we confirm that these solid
aggregates are β-sheet-rich and fibrillar in nature. These observations
not only add to the growing evidence that supports that RIM1α
and RBP can phase separate, but we also show that these proteins can
aggregate into fibrillar structures within condensates. Finally, we
find that in the presence of lipid vesicles, this liquid-to-solid
transition is suppressed, indicating the potential role that lipids
play in maintaining the liquid state of RIM1α/RBP condensates.
In the context of protein aggregation, these biophysical observations
report on the mechanisms behind the phase separation and subsequent
aggregation of RIM1α and RBP.