The Opitz syndrome gene product, MID1, associates with microtubules

Opitz syndrome (OS) is a genetically heterogeneous disorder characterized b y defects of the ventral midline, including hypertelorism, cleft lip and pa late, heart defects, and mental retardation. We recently identified the gen e responsible for X-linked OS. The ubiquitously expressed gene product, MID 1, is a member of the RING finger family, These proteins are characterized by an N-terminal tripartite protein-protein interaction domain and a conser ved C terminus of unknown function. Unlike other RING finger proteins for w hich diverse cellular functions have been proposed? the function of MID1 is as yet undefined. By using the green fluorescent protein as a tag, we show here that MID1 is a microtubule-associated protein that influences microtu bule dynamics in MID1-overexpressing cells. We confirm this observation by demonstrating a colocalization of MID1 and tubulin in subcellular fractions and the association of endogenous MID1 with microtubules after in vitro as sembly. Furthermore, overexpressed MID1 proteins harboring mutations descri bed in OS patients lack the capability to associate with microtubules, form ing cytoplasmic clumps instead. These data give an idea of the possible mol ecular pathomechanism underlying the OS phenotype.