LIPOCALINS, TRANSPORTERS OF HYDROPHOBIC M OLECULES

Lipocalins are a group of proteins present in both vertebrates and inv ertebrates, sharing a common function: they transport hydrophobic liga nds. 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) enclo sing an internal ligand binding site; (2) they are predominantly small secreted proteins (about 200 amino-acid residues) and possess a fe hi ghly conserved amino-acid positions; (3) the gene structure of seven l ipocalins presents surprising similarities; the size of the correspond ing exons are about the same and the positions of exons/introns juncti ons are well conserved through species. Furthermore, six out of eight lipocalin genes, in human, have been localized to the long arm of chro mosome 9. Given the tendency of the lipocalin genes to duplicate, it i s possible that a large series of duplications took place from a lipoc alin ancestral gene. Genes encoding lipocalins are expressed in a vari ety of tissues and, for all but two, the corresponding proteins are se creted in various fluids as blood serum, milk, saliva, tears, nasal mu cus, seminal fluid and amniotic fluid. Their exact role will be proven only after extensive structure-activity relationship studies.