title: Ammonia secretion from fish gill depends on a set of Rh glycoproteins.
abstract: Ammonia excretion from the gill in teleost fish is essential for nitrogen elimination. Although numerous physiological studies have measured ammonia excretion, the mechanism of ammonia movement through the membranes of gill epithelial cells is still unknown. Mammalian Rh glycoproteins are members of a family of proteins that mediate ammonia transport in bacteria, yeast, and plants. We identified the Rh glycoprotein homologs, fRhag, fRhbg, fRhcg1, and fRhcg2, of the pufferfish, Takifugu rubripes. Northern blot, in situ hybridization, and immunohistochemistry revealed that the pufferfish erythroid Rh glycoprotein homologue fRhag was present in red blood cells and the hematological organs (spleen and kidney) in fish. All four pufferfish Rh glycoproteins are specifically localized in the gill and line the pillar cells, pavement cells, and the mitochondrion-rich cells. Heterologous expression in Xenopus oocytes showed that they mediate methylammonium (an analog of ammonium) transport. These results suggest that pufferfish Rh glycoproteins are involved in ammonia excretion from the gill. These findings challenge the classic view that ammonia excretion in the fish gill occurs by passive diffusion.
authors.affiliation: Department of Biological Sciences, Tokyo Institute of Technology, Yokohama 226-8501, Japan.
authors.forename: Tsutomu
authors.initials: T
authors.lastname: Nakada
authors.forename: Connie M
authors.initials: CM
authors.lastname: Westhoff
authors.forename: Akira
authors.initials: A
authors.lastname: Kato
authors.forename: Shigehisa
authors.initials: S
authors.lastname: Hirose
mesh_terms: D000595:Amino Acid Sequence; D000641:Ammonia / Q000378:metabolism*; D000818:Animals; D001798:Blood Proteins / Q000737:chemistry*; D003001:Cloning, Molecular; D004912:Erythrocytes / Q000378:metabolism; D005399:Fishes; D005880:Gills / Q000378:metabolism*; D008562:Membrane Glycoproteins / Q000378:metabolism*; D008969:Molecular Sequence Data; D009584:Nitrogen / Q000378:metabolism; D009865:Oocytes / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D023741:Takifugu; D014981:Xenopus
chemical_list: D001798:Blood Proteins; D008562:Membrane Glycoproteins; D000641:Ammonia; D009584:Nitrogen