A novel uterine lipocalin supporting pregnancy in equids.

Equids, like horses and donkeys, produce a specific protein during early pregnancy that is believed to aid embryonic development. This protein, referred to as “P19,” bears resemblance to certain proteins found in mice, but also has unique features that classify it as a new member of the lipocalin protein family. The protein is likely responsible for transporting molecules essential for embryo formation. Identifying similar proteins in other mammals has so far been unsuccessful, but it’s suggested that this might be due to such proteins being produced in smaller quantities.

Identification of the P19 Protein

• Equids, including horses and donkeys, secrete a 19-kDa protein, known as P19, into the uterine lumen during early pregnancy.
• This protein is absorbed by the developing conceptus in significant quantities.
• P19 is a non-glycosylated and progesterone-dependent protein.

Structural Analysis of P19

• Structural analysis and sequencing have identified P19 as a member of the lipocalin family of proteins.
• P19 has the greatest similarity to lipocalins found in mouse urine proteins.
• However, P19 has unique structural traits, including a unique amino acid triplet within one of the invariant domains and an external tryptophan residue. These distinct features classify P19 as a new member of the lipocalin family.

Function of P19

• P19 is likely a transport protein involved in supporting early embryonic development as preliminary evidence suggests that it might transport a fatty acid or retinol-like molecule.

Exploration for P19 Homologues

• Though attempts to identify P19 homologues in other mammals have so far been unsuccessful, it is believed these proteins might exist but may be synthesized and secreted in smaller quantities, making them difficult to detect.

Significance of P19 in Equids

• Equids require large quantities of P19 protein during early pregnancy due to their species-specific biological processes including unusually late implantation and the presence of a capsule that surrounds the conceptus until about day 23 of gestation.