Unconventional Protein Functions through Liquid-Liquid Phase Separation in Stress Responses and Aging

Abstract

Just as human society relies on individuals specializing in distinct roles to ensure its proper functioning, a cell depends on organelles to coordinate essential biological processes. Traditionally, organelles have been defined as membrane-bound structures that establish distinct biochemical microenvironments through physical compartmentalization. However, in recent decades, the discovery and growing recognition of membraneless organelles, such as stress granules (SGs) and the pre-autophagosomal structure (PAS), have reshaped our understanding of intracellular organization and biological processes. These dynamic, non-membranous biomolecular condensates maintain their functional integrity through liquid-liquid phase separation (LLPS), a specialized phase transition in which a homogeneous solution spontaneously demixes into two immiscible liquid phases: a dense phase and a dilute phase. This biophysical process is driven by weak, multivalent interactions among macromolecules, such as proteins and nucleic acids. LLPS enables the reversible formation of spatially and functionally distinct compartments, allowing cells to dynamically regulate essential processes such as stress responses and aging, all without the need for lipid membranes. LLPS has thus opened a new dimension for scientific inquiry, enabling researchers to both observe and influence life’s fundamental processes.

This thesis provides new perspectives on traditionally well-studied proteins through the lens of LLPS, focusing on stress responses and aging. Specifically, it uncovers new roles for Lsm7 and thioredoxin reductase 1 (Trr1) in stress responses and aging, respectively, mediated through LLPS. For Lsm7, the mechanism of SG initiation via its phase separation, coupled with a conserved signaling pathway identified in this study, provides new insights into SG formation and their involvement in SG-associated human diseases. For Trr1, our findings reveal an unexpected connection between the autophagy process and the antioxidant system, with Trr1’s phase separation playing a key role in initiating ER-phagy during aging. These discoveries offer fresh perspectives on aging, age-related diseases, and the regulation of autophagy. Altogether, these findings offer new insights into fundamental biological processes and lay the groundwork for future research aimed at leveraging LLPS to better understand and potentially manipulate life itself.