This domain is found in mitochondrial ubiquitin ligase activator of NFKB 1 (MULAN, also known as MUL1) from animals and ubiquitin E3 Ligase SP1/SP2/SPL1/SPL2 from Arabidopsis.MUL1 is a multifunctional E3 ubiquitin ligase anchored in the outer mitochondrial membrane with its RING finger domain facing the cytoplasm. Mul1 functions as a ubiquitin ligase to ubiquitinate molecules such as mitofusin2 (Mfn2), Akt, p53 and ULK1, through its RING finger domain, leading to proteins degradation. Moreover, Mul1 can also act as a small ubiquitin-like modifiers (SUMO) E3 ligase to sumoylate proteins such as dynamin-related protein 1 (Drp1), enhancing protein stabilization []. It plays a role in the control of mitochondrial morphology, promotes mitochondrial fragmentation and influences mitochondrial localisation []. When over-expressed in human cells, it activates JNK through MAP3K7/TAK1 and induces caspase-dependent apoptosis []. MUL1 has also been shown to regulate RIG-I mediated antiviral response []. Ubiquitin E3 ligase SP1 associates with TOC (translocon at the outer envelope membrane of chloroplasts) complexes and mediates ubiquitination of TOC components, promoting their degradation. SP1-mediated regulation of chloroplast protein import contributes to the organellar proteome changes that occur during plant development []. It is also important for stress tolerance in plants [].
