Mitochondrial outer membrane protein which regulates mitochondrial organization. It is required for mitochondrial fission and promotes the recruitment and association of the fission mediator dynamin-related protein 1 (DNM1L) to the mitochondrial surface independently of the mitochondrial fission FIS1 and MFF proteins. Regulates DNM1L GTPase activity. Interacts with DNM1L. Colocalizes with DNM1L at mitochondrial membrane. Forms foci and rings around mitochondria. Does not bind ADP or other nucleotides, in contrast to MIEF1. Belongs to the MID49/MID51 family.