Exhibits weak E3 ubiquitin-protein ligase activity. E3 ubiquitin ligases accept ubiquitin from an E2 ubiquitin-conjugating enzyme in the form of a thioester and then directly transfer the ubiquitin to targeted substrates. Plays a role in the control of mitochondrial morphology by promoting mitochondrial fission. Negatively regulates the mitochondrial fusion protein marf by promoting its ubiquitination, acting in a pathway that is parallel to the park/pink1 regulatory pathway. S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Interacts with Marf. Auto-ubiquitinated. No gross morphological defects. Mitochondria are slightly elongated.