SEPTIN9 locally activates the RhoGEF ARHGEF18 to promote early stages of mitochondrial fission

Mitochondria continually undergo fission to maintain their network and health. Nascent fission sites are marked by the ER, which facilitates actin polymerization to drive calcium flux into the mitochondrion and constrict the inner mitochondrial membrane. Septins are a major eukaryotic cytoskeleton component that forms filaments that can both directly and indirectly modulate other cytoskeleton components, including actin. Septins have been implicated in mitochondrial fission; however, a connection between septins and the regulation of cytoskeletal machinery driving fission is not known. We find that SEPTIN9 is present at mitochondrial fission sites from its early stages with the ER and prior to the fission factor dynamin-related protein 1 (DRP1). SEPTIN9 has an isoform-specific role in fission, dependent on its N-terminal interaction to activate a Rho guanine nucleotide exchange factor, ARHGEF18. Without SEPTIN9, mitochondrial calcium influx is impaired, indicating SEPTIN9-containing octamers play a critical role in the early stages of fission.