Opitz syndrome (OS) is a multiple congenital anomaly manifested by abnormal closure of midline structures. The gene responsible for the X-linked form of this disease, MID1, encodes a protein (midin) that contains a RING, two B-boxes, a coiled-coil (the so-called tripartite motif) and an RFP-like domain. The tripartite motif is characteristic of a family of proteins, named the B-box family, involved in cell proliferation and development. Since the subcellular compartmentalization and the ability to form multiprotein structures both appear to be crucial for the function of this family of proteins, we have studied these properties on the wild-type and mutated forms of midin. We found that endogenous midin is associated with microtubules throughout the cell cycle, co-localizing with cytoplasmic fibres in interphase and with the mitotic spindle and midbodies during mitosis and cytokinesis. Immunoprecipitation experiments demonstrated the ability of the tripartite motif to mediate midin homodimerization, consistent with the evidence, obtained by gel filtration analysis, that midin exists in the form of large protein complexes. Functional characterization of altered forms of midin, resulting from mutations found in OS patients, revealed that association with microtubules is compromised, while the ability to homodimerize and form multiprotein complexes is retained. We suggest that midin is involved in the formation of multiprotein structures acting as anchor points to microtubules and that impaired association with these cytoskeletal structures causes OS developmental defects.
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