BPS2026 – Structural insights into regulation of mitochondrial fission

Mitochondrial dynamics has been increasingly implicated in the pathogenesis of various diseases including neurodegeneration and cancer, highlighting its potential as a therapeutic target. Mitochondrial fission eliminates diseased mitochondria, controls rates of energy production, and prepares mitochondria for cell division. Still, the molecular underpinnings of mitochondrial fission remain elusive in relation to membrane remodeling associated with these different cellular events. Previous studies have shown the key regulator of mitochondrial fission, dynamin-related protein 1 (Drp1), is a cytosolic GTPase that forms helical assemblies around mitochondrial sites marked for fission. Partner proteins such as mitochondrial fission 1 protein (Fis1) are important in recruitment of Drp1 to mitochondrial membranes. We are investigating the structural characteristics of both Fis1 and Drp1 to better understand the intermolecular interactions involved in mitochondrial fission. Drp1 has multiple disordered regions including the variable domain (VD), involved in lipid interactions, and the 80 loop, involved in Drp1 assembly. Isoforms of Drp1 have different levels of inclusion of the VD and the 80 loop, which provides a model for studying the role of disordered regions. Using a multifaceted approach, we observed that excluding regions of the VD leads to different affinities for mitochondrial lipids that affect Drp1 activity and assembly on lipid membranes. Our results suggest that inclusion of unstructured regions of Drp1 more tightly regulates fission to maximize metabolic activity associated with larger mitochondrial networks. In addition to Drp1 sequence changes, partner proteins modulate organelle fission. Here, we focus on Fis1, a conserved 16 kDa membrane-anchored receptor. While several structural studies have highlighted features of the cytosolic protein, we integrated full-length Fis1 into mitochondrial lipid and observed ordered membrane tubulation. This novel insight suggests Fis1 is not only recruiting Drp1 to the membrane, but constricting the membrane, potentially in preparation for Drp1 assembly.