A novel uterine lipocalin supporting pregnancy in equids

Horses,donkeys, and therefore, probably all equids, secrete a nonglycosylat ed, progesterone-dependent, 19-kDa protein (P19) into the uterine lumen dur ing early pregnancy, and significant quantities of it are taken up by the d eveloping conceptus. Sequence analysis and structural modelling have identi fied P19 as a lipocalin with greatest similarity to the murine major urinar y protein lipocalins. However, lack of strong identity with any particular group of lipocalins and several unusual structural features, including a un ique amino acid triplet within one of the invariant domains and an unusual external tryptophan residue, classify it as a new member of the lipocalin f amily. P19 is therefore likely to be a transport protein involved in suppor ting early embryonic development. Preliminary evidence using recombinant-de rived P19 and fluorescently tagged ligands suggests that it may transport a fatty acid or retinol-like molecule. Although an initial search failed to identify homologues of P19 in other mammals, they may nevertheless exist bu t are synthesised and secreted in much smaller quantities, making them diff icult to detect. Equids appear to need particularly large quantities of the protein during early pregnancy because of the unusually late implantation in this species and the presence of a capsule surrounding the conceptus unt il about day 23 of gestation.