Abstract :
[en] Proteins secreted by the placenta, when detected in the peripheral circulation of
the mother, can be useful indicators of both pregnancy and feto-trophoblast well-being (1-3).
In 1982, Butler et al. (4) isolated two pregnancy-specific proteins (PSPA and PSPB) from
bovine placental membranes. PSPA was identified as a-fetoprotein which is not strictly
limited to pregnancy, while PSPB was confirmed as placenta’s and pregnancy’s specific (5).
PSPB was characterized as a glycoprotein showing relative molecular masses (Mr) between
47 and 53 kDa and presenting different isoelectric points (from 4.0 to 4.4). The Mr of PSPB
was similar to the Mr of the molecule isolated by Laster in 1977 (6).
In 1991, Zoli et al. (7) purified a pregnancy-associated glycoprotein (PAG), later
designated PAG_1 and presently designated as PAG_I_ 67 considering its Mr. When isolated
from bovine fetal cotyledons PAG_I_67 was an acidic glycoprotein of 67 kDA. Four isoforms
(PI : 4.4, 4.6, 5.2 and 5.4) were detected in the initial preparation. Later molecular cloning
studies showed that the PSPB and PAG_I_67 were closely related in primary structure (8).
These glycoproteins (either PSPB and PAG_I_67) could be detected in the maternal
circulation at around the time that the trophoblast formed definitive attachment to the uterine
wall (figures A & B). Concentrations increased gradually therefore and reached peak values
just before parturition at about 1 to 5 mg/ml (9).The PSPB and PAG molecules are routinely
determined in peripheral maternal blood as pregnancy markers in cattle (5, 9,10).
Glycoproteins immunologically related to PAG_I_67 and PSPB have been
isolated and partially characterized from ovine fetal cotyledons: oPAG later designated
oPAG_I (11) and oPSPB (12). They also have been detected in maternal blood by week third
(12) or fourth (13) after breeding. Different forms (differing in Mr and isoelectric point) were
characterized after isolation from sheep cotyledons cultured in vitro (14).
Very recently, 3 different PAGs were characterized from goat placenta having Mr
of 55, 59 and 62 kDa. Each of them presented various isoeletric points (15).
In 1991, Xie et al., cloned PAG (now known as PAG_I_67) from late bovine and
ovine placenta by screening cDNA libraries with two anti-PAG antisera (16). The bovine and
ovine cDNAs encoding PAG_I shared 86% nucleotide sequence identic with one and other
and encoded proteins of 380 and 382 aminoacids respectively including a 15 aminoacid signal
sequence.
However, protein sequence data, (peptidic sequencing) already showed the first
aminoacid of the bovine PAG_I_67 was an arginine that corresponded to another one located
at position 39, downstream of the side of signal sequence cleavage, indicating that PAG_I_67
undergoes post-translational modifications from a proform.
