The functional roles of Liquid-Liquid Phase Separation inside the Golgi-matrix family of proteins

Biomolecular condensates, formed via liquid-liquid phase separation (LLPS), are instrumental in organising intracellular components into membrane-less organelles like Lewy bodies, the Corneal lens, and the Nucleolus. With increasing cell functions linked to LLPS, this research area is hailed as a new biology frontier. This presentation will discuss our recent findings on the significant presence of intrinsically disordered regions in Golgi matrix proteins and their correlation with the high tendency to form liquid-like condensates through LLPS in appropriate settings. I will particularly focus on Grh1, a crucial Golgi matrix protein involved in Golgi maintenance and critical for Unconventional Protein Secretion (UPS) pathways. UPS involves protein transport that bypasses the traditional endoplasmic reticulum (ER)/Golgi secretory machinery, typically triggered by stress conditions. We have found that Grh1's propensity to phase-separate in solution considerably intensifies under conditions critical for creating an elusive UPS vesicle intermediate, known as the "compartment of unconventional protein secretion" (CUPS). Our research indicates that Grh1's recruitment of UPS cargo occurs exclusively under phase-separated conditions, suggesting that CUPS might be membrane-less organelles enriched in Grh1. Additionally, we found that Grh1 has a high inclination to form amyloid-like structures under native conditions. However, we also discovered that LLPS protects Grh1 from transitioning into amyloid structures by precluding the formation of an intermediate state under physiological conditions. These observations exemplify a compelling instance of LLPS's dual role in dictating a protein's fate under differing cellular environments. Our findings enrich our understanding of LLPS's observable participation in biological processes and pave the way for future exploration of UPS mechanisms. For more information about our laboratory, please visit the website http://mendeslab.com.br/

This work was supported by São Paulo Research Foundation (2021/10465-7, 2017/24669-8).