UNCL, the mammalian homologue of UNC-50, is an inner nuclear membrane RNA-binding protein

We isolated a mammalian homologue of the C. elegans gene unc-50 that Lye ha ve named UNCL. The 777 kb rat UNCL cDNA encodes a 259 amino acid protein th at is expressed in a wide variety of tissues with highest mRNA levels in br ain, kidney and testis. Hydropathy plot analysis and in vitro translation e xperiments with microsomal membranes indicate that UNCL is a transmembrane protein. Hemagglutinin tagged UNCL was stably transfected into SaOS-2 osteo sarcoma cells and exhibited a nuclear rim staining pattern which was retain ed following extraction with 1% Triton X-100, suggesting that UNCL localize s to the inner nuclear membrane. UNCL-HA was extractable in 350 mM NaCl, su ggesting that UNCL is not associated with the nuclear matrix. Homopolymer R NA-binding assays performed on in vitro translated UNCL protein and 'struct ural modeling by homology' suggest that UNCL binds RNA via an amino-termina l RNA Recognition-like Motif. Since unc-50 is required for expression of as sembled muscle-type nicotinic receptors in the nematode we investigated whe ther UNCL had a similar function for mammalian nicotinic receptors. When UN CL was co-expressed with neural nicotinic receptors in Xenopus oocytes or C OS cells it increased expression of functional cell surface receptors up to 1.6-fold. We conclude that UNCL is a novel inner nuclear membrane protein that associates with RNA and is involved in the cell-surface expression of neuronal nicotinic receptors. UNCL plays a broader role because UNCL homolo gues are present in two yeast and a plant species, none of which express ni cotinic receptors and it is also found in tissues that lack nicotinic recep tors. (C) 2000 Elsevier Science B.V. All rights reserved.