Regulation of mitophagy by Fis1 and Fascin1-organized actin

Mitophagy, the autophagic degradation of mitochondria, plays a central role in controlling the quality and quantity of mitochondria, thereby ensuring cellular health. The mitochondrial outer membrane protein Fis1 is important for several types of mitophagy, but its mechanism of action remains unclear. F-actin is recruited to autophagic cargo and is important for autophagic progression, but the mechanism for its recruitment is poorly understood. To address the molecular function of Fis1, we performed affinity purification of Fis1 and mass spectrometry and identified the actin-bundling protein Fascin1 as a physical interactor. We demonstrate that Fis1 is required for recruitment of Fascin1 as well as F-actin to mitochondria under stress conditions, including mitochondrial depolarization and iron chelation. Iron chelation also triggers mitophagy that is independent of the Parkinson's associated gene Parkin, and we show that Fis1 enables recruitment of Fascin1-organized F-actin to facilitate proper morphogenesis of autophagosomes and the ensuing mitochondrial degradation. In contrast, although Parkin-mediated mitophagy also relies on Fis1, it is unaffected by loss of Fascin1 or F-actin recruitment. These findings indicate that Fis1 has distinct modes of action in mitophagy, depending on the triggering cellular stress. They establish Fis1 as a key driver of Fascin1 and F-actin recruitment to mitochondria, events that are critical for autophagosome morphogenesis during iron-chelation-induced mitophagy.