Uterocalin: a protein uniquely specialised to support mobile encapsulated embryos of horses, rhinos and tapirs

Equids are considered unique in having encapsulated embryos that are actively moved between the uteri of mares during the first two and three weeks of pregnancy and do not form a placenta until about 40 days, the longest pre-placentation period known amongst Eutheria. Their embryos are likely supplied through the glycoprotein capsule with selected nutrients by uterocalin, a carrier protein secreted by maternal endometrial glands. Uterocalin binds polyunsaturated fatty acids and retinol (Vitamin A), concentrates in the capsule, and is internalised by the embryo. Here it is shown that the structures of equid uterocalin comprise an excess of metabolism-related essential amino acids, and structural specialisations potentially involved in stabilising the capsule matrix until emergence and engagement with a receptor on the trophoblast surface. The genomes of other perissodactyls (rhinos and tapirs) encode uterocalins with amino acid sequences, excesses of essential amino acids, and predicted structural properties, all closely similar to horse uterocalin. These characteristics are unique to the perissodactyl uterocalins and are not found in similar proteins of other mammal groups. Hence, rhinos and tapirs probably have equid-like reproductive specialisations retained since before the Cretaceous-Paleogene mass extinction and that uterocalin is key to both embryo nutrition and capsule formation.