title: Transmembrane topology of PiT-2, a phosphate transporter-retrovirus receptor.
abstract: PiT-1 and PiT-2 are related multiple transmembrane proteins which function as sodium-dependent phosphate transporters and as the cell receptors of several oncoretroviruses. Two copies of a homology domain that is found in distantly related species assign these proteins to a large family of phosphate transporters. A current membrane topology model of PiT-1 and PiT-2 predicts 10 transmembrane domains. However, the validity of this model has not been addressed experimentally. We addressed this issue by a comprehensive study of human PiT-2. Evidence was obtained for glycosylation of asparagine 81. Epitope tagging showed that the N- and C-terminal extremities are extracellular. The orientation of C-terminal-truncation mutants expressed in cell-free translation assays and incorporated into microsomal membranes was examined by immunoprecipitation. Data were interpreted with respect to previous knowledge about retrovirus binding sites, to the existence of repeated homology domains, and to predictions made in family members. A model in which PiT-2 has 12 transmembrane domains and extracellular N- and C-terminal extremities is proposed. This model, which differs significantly from previous predictions about PiT-2 topology, may be useful for further investigations of PiT-2 interactions with other proteins and for the understanding of PiT-2 transporter and virus receptor functions.
authors.affiliation: Laboratoire Rétrovirus et Transfert Génétique, CNRS URA 1930, Institut Pasteur, 75724 Paris, France.
authors.forename: C
authors.initials: C
authors.lastname: Salaün
authors.forename: P
authors.initials: P
authors.lastname: Rodrigues
authors.forename: J M
authors.initials: JM
authors.lastname: Heard
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001216:Asparagine / Q000378:metabolism; D002352:Carrier Proteins / Q000737:chemistry* / Q000378:metabolism*; D019295:Computational Biology; D000939:Epitopes; D006031:Glycosylation; D006801:Humans; D051379:Mice; D008954:Models, Biological; D008969:Molecular Sequence Data; D009154:Mutation; D011233:Precipitin Tests; D014176:Protein Biosynthesis; D051381:Rats; D011991:Receptors, Virus / Q000378:metabolism*; D012190:Retroviridae / Q000378:metabolism*; D050608:Sodium-Phosphate Cotransporter Proteins; D050611:Sodium-Phosphate Cotransporter Proteins, Type III; D027981:Symporters
chemical_list: D002352:Carrier Proteins; D000939:Epitopes; D011991:Receptors, Virus; C493586:SLC20A1 protein, human; C493589:SLC20A2 protein, human; C493587:Slc20a1 protein, mouse; C493588:Slc20a1 protein, rat; C493565:Slc20a2 protein, mouse; D050608:Sodium-Phosphate Cotransporter Proteins; D050611:Sodium-Phosphate Cotransporter Proteins, Type III; D027981:Symporters; D001216:Asparagine