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  • Mariana J. Do Amaral

Human Brain Expressed X-linked 3 (hBEX3) in the spotlight of phase transitions

Updated: Jul 31, 2020

Protein Liquid-liquid phase transition

Human BEX3 is a disordered protein that assembles into liquid condensates in vitro (left). However, it co-purifies with small fragments of tRNA (tRFs) which confer well-defined 3D fold and a high number of small condensates (right). Over time, hBEX3 samples (kept in physiological buffer at 4 °C) phase transits to aggregates with morphology tuned by the presence of tRFs (fibrillary) or absence (amorphous). Aggregation is increased in absence of tRFs, and in cells the tRFs might function as molecular chaperones preventing misfolded higher-order species of hBEX3.

This research represents a major leap-forward on understanding hBEX3 structural features enlightening the property of tRFs binding and phase transitions in a linked way with proteins long implicated in neurodegenerative aggregation

Researchers at the Federal University of Rio de Janeiro (UFRJ), in association with a researcher of the Ohio State University, identified that human Brain Expressed X-linked 3 (hBEX3), a protein highly expressed in brain with unclear function, over time aggregates into amyloid structures resembling the brain deposits found in neurodegenerative diseases. Despite being an intrinsically disordered oligomer, hBEX3 adopts a well-defined 3D monomeric structure in presence of tRNA fragments (tRFs). In addition to the dramatic impact on hBEX3 folding, tRFs are able to modulate liquid-liquid phase separation, the principle behind membranelles organelles assembly, and tRFs decrease amyloid formation. These findings advocate paramount importance towards understanding BEX family involvement in neurodevelopment and cell death. The study led by the PhD student Mariana Juliani Amaral and Professors Marcius S. Almeida and Katia M.S. Cabral was published at Journal of Molecular Biology.

Human cells have around 20,000 proteins, among them there are five from BEX family that stand out for being highly expressed in the brain. Until the 90’s it was believed that only “functional” proteins adopt well defined three-dimensional structure, which was responsible for their role - the traditional structure-function paradigm. However, it is well stablished that one-third of all proteins account for 'intrinsically disordered proteins' that do not adopt fixed structures, instead, behave as highly dynamic structural ensembles and are important hubs of cell signaling regulation. Human Brain Expressed-X linked 3 (hBEX3) is predicted to be intrinsically disordered and has a controversial role on promoting cell death or survival. Biochemists at UFRJ sought to understand hBEX3 molecular characteristics using a multitude of biochemical and biophysical techniques. Interestingly, they showed that hBEX3 has well-defined compact 3D structure in the presence of small fragments of tRNA (tRFs). Conversely, in absence of tRNA hBEX3 presented an elongated disordered oligomeric structure. Recent work have shown that RNA-binding disordered proteins, long associated to neurotoxic aggregation, also drive liquid condensates with specialized functions in cells like membranelles organelles. Misregulation of condensates formation turn liquid condensates into solid-like structures associated with pathologies. In fact, human BEX3 self-associates to form liquid-like condensates in vitro drove by electrostatic and weak hydrophobic forces. In the presence of tRFs, hBEX3 spherical condensates are numerous and smaller, highlighting a possible role of tRNA on modifying hBEX3 condensates material and biochemical properties. In absence of tRNA, hBEX3 phase separates in a gel-like phase formed by reversible amyloids, since temperature increment was able to dissolve the aggregates and even induce liquid transition.

The paper "Phase Separation and Disorder-to-Order Transition of Human Brain Expressed X-Linked 3 (hBEX3) in the Presence of Small Fragments of tRNA" is available online on Journal of Molecular Biology (doi: 10.1016/j.jmb.2020.02.030).

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