Molecular cloning of Rhombex-40 a transmembrane protein from the ventral medullary surface of the rat brain by differential display

Respiration-related neurons, which detect various chemicals in cerebrospina l fluid, are localized to the ventral medullary surface (VMS). We hypothesi zed that expression of genes involved in respiratory function is upregulate d in the VMS. By differential display, we looked for genes differentially e xpressed in VMS neurons and cerebral cortical neurons. Seventeen clones of interest were isolated, and sequence analysis revealed that one of these cl ones encoded a putative transmembrane protein, rhombencephalic expression p rotein-40 kDa (Rhombex-40). The rat Rhombex-40 was composed of 374 amino ac id residues, and the predicted secondary structure displays a signal peptid e in the N-terminus and single-pass transmembrane domain in the center of t he sequence. An analysis of consensus sequences identified several phosphor ylation sites in the intracellular domain. Expression of rat Rhombex-40 mRN A is high in the brain, and low in lung, liver and kidney. No homologous pr otein sequence was found in database searches. Whereas the biological funct ion of this protein is presently unknown, its structural features and high expression in the brain suggest that Rhombex-40 may function as a novel tra nsmembrane molecule in neural cells of the brain.