A NOVEL UTERINE PROTEIN THAT ASSOCIATES WITH THE EMBRYONIC CAPSULE INEQUIDS

An apparently unique protein produced in large quantities by the endom etrium of the mare which adheres to, or is incorporated into, the acel lular capsule that surrounds the equine conceptus in early pregnancy, has been characterized and partially sequenced. It has a molecular mas s of approximately 18 kDa on SDS-PAGE gels and is nonglycosylated as a ssessed by a sensitive carbohydrate detection kit. Comparison of its f irst 24 amino-terminal amino acids with all entries in the databases f ailed to show any significant identity with any other protein sequence . Secretion of the protein appears to be progesterone dependent, as it s presence in uterine flushings correlates with peripheral serum proge sterone profiles during the oestrous cycle and its secretion can be in duced in anoestrous mares by administration of a synthetic progestagen . However, in pregnant mares, the protein disappears from the uterus a fter about day 20 (term = 320-340 days), despite the persistence of hi gh serum concentrations of progesterone, indicating that additional me chanisms control its synthesis and secretion. The strong association o f the protein with the glycoprotein capsule that surrounds the equine blastocyst suggests that it may be incorporated into the capsule as th e capsule expands from day 11 after ovulation. Alternatively, or addit ionally, it may be involved in the transport of nutrients or other sub stances through the capsule, and may therefore play an important role in the maintenance of pregnancy.