Sheep blastocysts (Day 13-21) incubated in a modified Minimum Essential Medium released proteins into the medium at an approximately linear rate over a 24-h period. Single Day-16 blastocysts converted 2-8% of the radioactivity supplied (100 muCi L-[3H]leucine) into non-dialysable macromolecules which were released into the medium. Two-dimensional polyacrylamide gel electrophoresis revealed that at Day 13 there was only one major product (Protein X), consisting of three closely similar isoelectric species of (pI of denatured polypeptides about 5.5), each with molecular weights of 17 000. Between Days 14 and 21 additional proteins were detected. One of these was of high molecular weight (greater than 660 000) and did not appear on the two-dimensional gels, but Protein X continued to predominate until Day 23 when it could not be detected. Explants of chorion from Day 30 of pregnancy failed to secrete Protein X. Protein X was released in significant quantities (50-100 micrograms per 24 h) by the trophoblast but not the yolk sac of Day-14 and Day-16 conceptuses, but was present in very low amounts in the tissues. Protein X from the incubation medium of Day-14 and Day-16 conceptuses was purified by successive DEAE ion exchange and Sephacryl S-200 gel chromatography. Because Protein X and some of the other proteins are produced transiently between Days 13 and 21, it is possible that they may play a role in maternal recognition of pregnancy in sheep.
