Lipocalines et transport des ligands hydrophobes
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Lipocalins are a group of proteins present in both vertebrates and invertebrates, sharing a common function: they transport hydrophobic ligands. There are three main criteria suggesting that they belong to the same family: (1) the three dimensional structure of five of them show eight beta strands which form a calyx (antiparallel beta barrel) enclosing an internal ligand binding site; (2) they are predominantly small secreted proteins (about 200 amino-acid residues) and possess a fe highly conserved amino-acid positions; (3) the gene structure of seven lipocalins presents surprising similarities; the size of the corresponding exons are about the same and the positions of exons/introns junctions are well conserved through species. Furthermore, six out of eight lipocalin genes, in human, have been localized to the long arm of chromosome 9. Given the tendency of the lipocalin genes to duplicate, it is possible that a large series of duplications took place from a lipocalin ancestral gene. Genes encoding lipocalins are expressed in a variety of tissues and, for all but two, the corresponding proteins are secreted in various fluids as blood serum, milk, saliva, tears, nasal mucus, seminal fluid and amniotic fluid. Their exact role will be proven only after extensive structure-activity relationship studies.
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Gachon, A.M.F., Lipocalines et transport des ligands hydrophobes, Med Sci (Paris), 1994, Vol. 10, N° 1; p.22-29