The carcinoembryonic antigen family
Kammerer and Zimmermann BMC Biology 2010, 8:12
Human placenta secretes great quantities of protein including pregnancy-specific beta1-glycoprotein (PSG). The precise biological function of PSG has yet to be resolved, although it is thought to modify maternal immune responses.
PSGs are part of the carcinoembryonic antigen (CEA) family, other members being the CEA cell adhesion molecules (CEACAMs). Primordial CEACAMs occur across the full range of mammals, including marsupials and monotremes (green shading in the figure).
Interestingly, there has been convergent evolution of PSGs in rodents and primates (grey shading). They are not found in dogs or cattle although they may occur in the horse.
These proteins belong to the immunoglobulin gene superfamily. Human PSGs are composed of an immunoglobulin variable-like domain (red circle) and a variable number of constant-like domains (blue circles). As the figure shows, mouse PSGs are built slightly differently. This supports the idea that PSGs evolved separately on the rodent and primate lineages.
Towards the end of human pregnancy, PSG is the most abundant placental protein in maternal blood. It is surprising we know so little about it.