Uterocalin, a lipocalin provisioning the preattachment equine conceptus: fatty acid and retinol binding properties, and structural characterization

The equine conceptus is surrounded by a fibrous capsule that persists until about day 20 of pregnancy, whereupon the capsule is lost, the conceptus at taches to the endometrium and placentation proceeds. Before attachment, the endometrium secretes in abundance a protein of the lipocalin family, utero calin. The cessation of secretion coincides with the end of the period duri ng which the conceptus is enclosed in its capsule, suggesting that uterocal in is essential for the support of the embryo before direct contact between maternal and foetal tissues is established. Using recombinant protein and fluorescence-based assays, we show that equine uterocalin binds the fluores cent fatty acids 11-(dansylamino)undecanoic acid, dansyl-D,L-alpha -amino-o ctanoic acid and cis-parinaric acid, and, by competition, oleic, palmitic, arachidonic, docosahexaenoic, gamma -linolenic, cis-eicosapentaenoic and li noleic acids. Uterocalin also binds all-trans-retinol, the binding site for which is coincident or interactive with that for fatty acids. Molecular mo delling and intrinsic fluorescence analysis of the wild-type protein and a Trp --> Glu mutant protein indicated that uterocalin has an unusually solve nt-exposed Trp side chain projecting from its large helix directly into sol vent. This feature is unusual among lipocalins and might relate to binding to, and uptake by, the trophoblast. Uterocalin therefore has the localizati on and binding activities for the provisioning of the equine conceptus with lipids including those essential for morphogenesis and pattern formation. The possession of a fibrous capsule surrounding the conceptus might be an a ncestral condition in mammals; homologues of uterocalin might be essential for early development in marsupials and in eutherians in which there is a p rolonged preimplantation period.