Involved in the fragmentation of the mitochondrial network and its perinuclear clustering. Plays a minor role in the recruitment and association of the fission mediator dynamin-related protein 1 (DNM1L) to the mitochondrial surface and mitochondrial fission. Can induce cytochrome c release from the mitochondrion to the cytosol, ultimately leading to apoptosis. Also mediates peroxisomal fission (By similarity). Interacts with DNM1L/DLP1 through the TPR region. Interacts with MARCHF5. Interacts with MIEF1. Interacts with PEX11A, PEX11B and PEX11G. The C-terminus is required for mitochondrial or peroxisomal localization, while the N-terminus is necessary for mitochondrial or peroxisomal fission, localization and regulation of the interaction with DNM1L. Ubiquitinated by MARCHF5. Belongs to the FIS1 family.