title: Protein synthesis at the blood-brain barrier. The major protein secreted by amphibian choroid plexus is a lipocalin.
abstract: Among the proteins secreted by choroid plexus of vertebrates, one protein is much more abundant than all others. In mammals, birds, and reptiles this protein is transthyretin, a tetramer of identical 15-kDa subunits. In this study choroid plexus from frogs, tadpoles, and toads incubated in vitro were found to synthesize and secrete one predominant protein. However, this consisted of one single 20-kDa polypeptide chain. It was expressed throughout amphibian metamorphosis. Part of its amino acid sequence was determined and used for construction of oligonucleotides for polymerase chain reaction. The amplified DNA was used to screen a toad choroid plexus cDNA library. Full-length cDNA clones were isolated and sequenced. The derived amino acid sequence for the encoded protein was 183 amino acids long, including a 20-amino acid presegment. The calculated molecular weight of the mature protein was 18,500. Sequence comparison with other proteins showed that the protein belonged to the lipocalin superfamily. Its expression was highest in choroid plexus, much lower in other brain areas, and absent from liver. Since no transthyretin was detected in proteins secreted from amphibian choroid plexus, abundant synthesis and secretion of transthyretin in choroid plexus must have evolved only after the stage of the amphibians.
authors.affiliation: Russell Grimwade School of Biochemistry, University of Melbourne, Parkville, Victoria, Australia.
authors.forename: M G
authors.initials: MG
authors.lastname: Achen
authors.forename: P J
authors.initials: PJ
authors.lastname: Harms
authors.forename: T
authors.initials: T
authors.lastname: Thomas
authors.forename: S J
authors.initials: SJ
authors.lastname: Richardson
authors.forename: R E
authors.initials: RE
authors.lastname: Wettenhall
authors.forename: G
authors.initials: G
authors.lastname: Schreiber
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D001812:Blood-Brain Barrier / Q000502:physiology*; D015152:Blotting, Northern; D002024:Bufo marinus; D002831:Choroid Plexus / Q000254:growth & development / Q000378:metabolism*; D002850:Chromatography, Gel; D004247:DNA / Q000235:genetics / Q000302:isolation & purification; D015723:Gene Library; D006801:Humans; D019746:Intramolecular Oxidoreductases; D007535:Isomerases / Q000235:genetics*; D054834:Lipocalins; D008675:Metamorphosis, Biological; D051379:Mice; D008969:Molecular Sequence Data; D008970:Molecular Weight; D009419:Nerve Tissue Proteins / Q000009:adverse effects / Q000096:biosynthesis* / Q000235:genetics / Q000378:metabolism; D009928:Organ Specificity; D012313:RNA / Q000235:genetics / Q000302:isolation & purification; D015183:Restriction Mapping; D017386:Sequence Homology, Amino Acid
chemical_list: D054834:Lipocalins; D009419:Nerve Tissue Proteins; C077950:major protein, amphibian choroid plexus; D012313:RNA; D004247:DNA; D007535:Isomerases; D019746:Intramolecular Oxidoreductases; C022466:prostaglandin R2 D-isomerase