Cloning of a Human Epididymis-Specific mRNA, HE6, Encoding a Novel Member of the Seven Transmembrane-Domain Receptor Superfamily

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Abstract
A novel gene product, HE6, showing homology to the seven transmembrane-domain (Tm7) receptor super-family, has been cloned by differential screening from a human epididymal cDNA library. The cDNA clone represented an abundant approximately 5-kb mRNA, comprising 0.01% of the cDNA library. Northern blot analysis including various human tissues revealed an epididymis-specific expression. In situ transcript hybridization localized the mRNA within the epithelial cells lining the epididymal duct. Southern blot analysis, employing a fragment encoding part of the amino-terminal extracellular domain as a probe, identified an autosomal single-copy gene in the human genome. Homologous cDNA products showing 90% sequence identity were observed in the epididymides of all mammalian species investigated. A cloning and sequencing strategy, combining approximately 3.7-kb cDNA fragments obtained by conventional cDNA library construction with overlapping 5′ rapid amplification of cDNA ends (RACE) fragments, yielded total sequence information of 4.7 kb for the human mRNA. This sequence comprises a long open reading frame of 3.1 kb. A homology search for related sequences revealed highest similarity (25% amino acid identity) with the secretin/vasoactive intestinal peptide (VIP) superfamily of G-protein-coupled receptors. The predicted extracellular aminoterminal extension, however, was much longer than in the other members, and showed similarity to highly glycosylated mucin-like cell-surface molecules.