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 cerebrospinal fluid, are localized to the ventral medullary surface (VMS). We hypothesized that expression of genes involved in respiratory function is upregulated in the VMS. By differential display, we looked for genes differentially expressed in VMS neurons and cerebral cortical neurons. Seventeen clones of interest were isolated, and sequence analysis revealed that one of these clones encoded a putative transmembrane protein, rhombencephalic expression protein-40 kDa (Rhombex-40). The rat Rhombex-40 was composed of 374 amino acid residues, and the predicted secondary structure displays a signal peptide in the N-terminus and single-pass transmembrane domain in the center of the sequence. An analysis of consensus sequences identified several phosphorylation sites in the intracellular domain. Expression of rat Rhombex-40 mRNA is high in the brain, and low in lung, liver and kidney. No homologous protein sequence was found in database searches. Whereas the biological function of this protein is presently unknown, its structural features and high expression in the brain suggest that Rhombex-40 may function as a novel transmembrane molecule in neural cells of the brain.