RING finger, HC subclass, found in midline-1 (MID1) and similar proteins
MID1, also known as midin, midline 1 RING finger protein, putative transcription factor XPRF, RING finger protein 59 (RNF59), or tripartite motif-containing protein 18 (TRIM18), is a microtubule-associated E3 ubiquitin-protein ligase implicated in epithelial-mesenchymal differentiation, cell migration and adhesion, and programmed cell death along specific regions of the ventral midline during embryogenesis. It monoubiquinates the alpha4 subunit of protein phosphatase 2A (PP2A), promoting proteosomal degradation of the catalytic subunit of PP2A (PP2Ac) and preventing the A and B subunits from forming an active complex. It promotes allergen and rhinovirus-induced asthma through the inhibition of PP2A activity. It is strongly upregulated in cytotoxic lymphocytes (CTLs) and directs lytic granule exocytosis and cytotoxicity of killer T cells. Loss-of-function mutations in MID1 lead to the human X-linked Opitz G/BBB (XLOS) syndrome characterized by defective midline development during embryogenesis. MID1 belongs to the C-I subclass of TRIM (tripartite motif) family of proteins that are defined by their N-terminal RBCC (RING, Bbox, and coiled coil) domains, including three consecutive zinc-binding domains, a C3HC4-type RING-HC finger, Bbox1 and Bbox2, and a coiled coil region, as well as a COS (carboxyl-terminal subgroup one signature) box, a fibronectin type III (FN3) domain, and a B30.2/SPRY (SplA and ryanodine receptor) domain positioned C-terminal to the RBCC domain. MID1 hetero-dimerizes in vitro with its paralog MID2.