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Changes in major proteins in the embryonic capsule during immobilization (fixation) of the conceptus in the third week of pregnancy in the mare

¹ Departments of Pathobiology and ² Biomedical Sciences, Ontario Veterinary College, University of Guelph, Guelph, Ontario, N1G 2W1, Canada ³ Division of Environmental and Evolutionary Biology, Institute of Biomedical and Life Sciences, University of Glasgow, Graham Kerr Building, Glasgow G12 8QQ, Scotland, UK

Correspondence should be addressed to M A Hayes; Email: ahayes{at}uoguelph.ca

During the third week of pregnancy, the equine conceptus is enclosed within a capsule, the glycan composition of which changes at around day 16 (ovulation = day 0) when the conceptus becomes immobilized (fixed) in the uterine lumen. Our objective was to characterize the process of fixation by identifying changes in major capsule-associated proteins. Individual equine conceptuses (n = 55) were collected transcervically by uterine lavage between days 13.5 and 26.5. Major proteins extracted from capsules were compared with those in fluids from the uterus and yolk sac by SDS–PAGE. Until day 14, a major capsule-associated protein that migrated at ~10 kDa was identified by N-terminal sequencing as equine ß2 microglobulin (ß2M). During fixation, ß2M in the capsule underwent limited proteolysis to an ~8 kDa form lacking nine amino acids from the N terminus, and was subsequently degraded. Expression of ß2M mRNA was detected in the yolk-sac wall tissues and endometrium between days 13.5 and 17.5. During this period, ß2M in the capsule was evidently not part of a complex with major histocompatibility complex class 1 heavy chain bands because these were undetectable in the capsule and uterine lavage. Uterocalin (p19) was detected in uterine lavage and capsule throughout fixation, but in yolk-sac fluid only before fixation. These studies indicate that intact ß2M is a major protein associated with the embryonic capsule before fixation, after which it undergoes limited proteolysis to a truncated ~8 kDa form that remains in the capsule after the conceptus is immobilized.