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B-box-type 2 zinc finger found in midline (MID) family The MID family includes MID1 and MID2. 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. MID2, also known as midin-2, midline defect 2, RING finger protein 60 (RNF60), or tripartite motif-containing protein 1 (TRIM1), is highly related to MID1. It associates with the microtubule network and may at least partially compensate for the loss of MID1. Both MID1 and MID2 interacts with Alpha 4, which is a regulatory subunit of PP2-type phosphatases, such as PP2A, and an integral component of the rapamycin-sensitive signaling pathway. They also play a central role in the regulation of granule exocytosis, and functional redundancy exists between MID1 and MID2 in cytotoxic lymphocytes (CTL). Both MID1 and MID2 belong 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 RING 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. The type 2 B-box (Bbox2) zinc finger is characterized by a CHC3H2 zinc-binding consensus motif.
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