Biomolecular Condensates as Potential Players in Protein Secretion and Golgi Organization: Insights from GRASP Proteins

The role of biomolecular condensates formed via phase separation in cellular organization is gaining recognition as a key mechanism in stress adaptation, gene regulation, cellular signalling, among others. This study investigates phase separation in conditions relevant for two critical cellular processes: unconventional protein secretion (UPS) and the structural organization of the Golgi apparatus. In the first part, we demonstrated that under starvation-mimicking conditions, the yeast GRASP homologue, Grh1, undergoes phase separation (PS) to form condensates, which we propose as being the Compartment for Unconventional Protein Secretion (CUPS) or its precursor. These Grh1 condensates can recruit UPS cargo and exhibit dynamic liquid-to-solid transitions, which could support cellular adaptation to stress. The second part of our study focused on the human Golgi matrix proteins GRASP55 and GRASP65 and their capacity to form condensates. GRASP55, in particular, demonstrated a strong tendency for PS, which is modulated by pH and weak hydrophobic interactions, that could contribute to the dynamic self-organization and functional integrity of the Golgi apparatus as well as the UPS. On the other hand, GRASP65 does not form condensate in functionally relevant scenarios. Together, these findings suggest GRASP phase separation as a conserved mechanism across eukaryotic cells, playing potential roles in stress-responsive protein secretion and organelle architecture. This work aims to bridge the understanding of phase separation in membrane-less and membrane-bound organelles, offering insights into cellular resilience and organization.